fix(snmp-discovery): discover optics published as container or port rows

docs/backends/snmp_discovery/supported_platforms.md

@@ -137,3 +137,19 @@ Any other vendor that populates `entPhysicalTable` per RFC 6933 will be discover
 
 **PID classifier.** Module rows (`entPhysicalClass = module(9)`) are split into `supervisor` / `linecard` / `transceiver` / `psu` / `fan` types by matching `entPhysicalModelName` (the vendor product ID) against a small set of prefix rules: `SUP*` / `SUPV*` / `SUP\d` → `supervisor`; optic prefixes `SFP-` / `QSFP-` / `X2-` / `GLC-` / `CFP-` / `XENPAK-` / `XFP-` → `transceiver`; `PSU-` / `PWR-` or `-PWR-` infixes → `psu`; `FAN` / `-FAN-` → `fan`; everything else inside a chassis slot defaults to `linecard`. The classifier is shared across all vendors — no Cisco-only / Arista-only branch. PSU and fan modules are recognised so they label correctly in OTLP metrics, but **never** emitted as `Module` entities (counted in `modules_dropped` instead) — the inventory surface in NetBox stays scoped to line cards, supervisors, and transceivers.
 
+### Fixed-port optics
+
+Not every transceiver arrives as a `module(9)` row under a linecard. On fixed-port hardware — and in some per-port cages on modular platforms — the optic itself is published as a `container(5)` row or a `port(10)` row, with no `module(9)` level in between. The module scan widens to those two classes, but only for rows that carry a recognised optic PID (the same prefix list the PID classifier above uses); a bare cage or a port row without one is left alone, so ordinary container and port rows are never mistaken for modules. `entPhysicalClass = module(9)` remains the only class scanned unconditionally.
+
+**Bay naming.** A fixed-port optic's bay is named for the interface it serves rather than the cage's bare position number, which by itself does not identify the port. The name is taken from an anchored `Xcvr for <iface>` `entPhysicalDescr` — anchored because some platforms publish a "Lane N for Xcvr for `<iface>`" row beneath the same optic, and an unanchored match would emit one bay per lane — or, failing that, from an interface-shaped `entPhysicalName`. Either candidate must contain a digit: one platform names every optic row with the literal token `port`, which would otherwise give every bay on the chassis the same name.
+
+**Optics without a serial.** A module bay requires a serial on the module installed in it, so a `container(5)` or `port(10)` optic reporting none is discovered but never emitted — its would-be bay and model are dropped along with it. This rule is scoped to those two classes, not `module(9)`: a `module(9)` optic under a linecard with no serial was already discoverable before the `container(5)`/`port(10)` widening above — nested under its linecard and emitted with its serial left unset — and applying the same gate to it would have removed working behaviour rather than tightened new behaviour, so it does not. Skipped `container(5)`/`port(10)` optics are aggregated into a single warning per device rather than one line per optic, so a platform publishing dozens of serial-less rows doesn't flood the log every poll.
+
+**Interface-association limitation.** A fixed-port transceiver — one with no `module(9)` parent — is still emitted as its own `ModuleBay` and `Module`, with model, serial, and the interface-named bay all present. Its owning interface does not carry a `module=` reference, though: the `entAliasMappingTable`-based routing that attaches `Interface.module` only walks transceivers nested under another module, and a fixed-port optic never is one, so it never participates — this holds even on a device that populates `entAliasMappingTable`. The visible shape is the same as the documented `iosxr` limitation on the device-discovery side (a transceiver Module emitted without its interface backref), though the cause there is a location-string/interface-name mismatch rather than this routing gap.
+
+**Known false negatives.** The PID prefix list is not exhaustive. Real transceivers observed in captures whose PID it does not match, and are therefore not recognised as transceivers: `CAB-SFP-SFP-1M`, `SFPP-PC005`, `ABCU-5710RZ-CS5`, `FN-TRAN-SFP+GC`, `CVR-QSFP-SFP10G`, `10GE SR 300m SFP+`, `10GE LR 10km SFP+`. Broadening the list is deliberately deferred: those prefixes now reach `container(5)` and `port(10)` rows as well as `module(9)` ones, so loosening them enough to catch these would risk matching a non-optic row too and manufacturing a bay that doesn't exist on the device.
+
+**Empty-bay harvest and nested containers.** A `container(5)` row with no `module(9)` or `port(10)` child underneath is harvested as an empty bay in `full` mode (see [Empty bays](./README.md#modules--modulebays) in the SNMP discovery README). That harvest used to mark only a bay's *nearest* `container(5)` ancestor as populated, so a container whose own children are themselves containers — never a module or port leaf directly — was reported as an empty bay even when everything beneath it was fully populated. On two Arista EOS captures this misreported three containers per device — the transceiver, fan-tray, and power-supply slot containers — as empty even though every cage beneath them held a populated module. The rule is now: a container is an empty bay only if nothing beneath it was emitted, implemented by walking up from every populated bay and marking each `container(5)` ancestor in turn. A genuinely empty slot is unaffected: it has no module beneath it at any depth, so it is still correctly reported as an empty bay.
+
+**Test fixtures.** With one exception, the unit fixtures behind these rules are transcribed from real SNMP simulator recordings rather than hand-authored: row class, parentage, `entPhysicalParentRelPos`, and which fields are populated all mirror the capture, because those are exactly the values the logic above reads. Two mirror separate Arista EOS fixed-port captures — one where every optic has a lane child, one where almost none do, which is the shape that would otherwise reach the empty-bay harvest. One mirrors a Cisco Catalyst 9404R capture with a `port(10)` optic nested inside a linecard's own cage. One mirrors a two-member stack whose optics are named with the literal token `port`. One mirrors a fixed-port device reporting no serial on any optic. The single exception is synthetic rather than transcribed: no captured device has been seen publishing a serial-less optic in a `container(5)` cage of its own, so that shape is built by hand to pin the invariant that such a cage still gets harvested as an empty bay once the serial-less optic above it is dropped.
+