feat(imgur): read base and auth URLs from service config

[alexeyqu]

Makes ImgurTransferExtension's API root and ImgurOAuthConfig's authorization and token endpoints configurable, so the adapter can be pointed at a staging environment or a test double without rebuilding it.

All three currently hardcode Imgur's own URLs. They now read config/imgur.yaml from the classpath, falling back to exactly today's values when no such file exists:

serviceConfig:
  baseUrl:  "https://api.imgur.com/3"
  authUrl:  "https://api.imgur.com/oauth2/authorize"
  tokenUrl: "https://api.imgur.com/oauth2/token"

Behaviour is unchanged for every distribution in this repo — none ships a config/imgur.yaml, so every code path falls through to the defaults.

Stack

PR	Scope
#1509 (feat/offline-demo-exporter)	offline-demo becomes a complete export/import pair
#1516 (test/e2e-offline-demo-compose)	the compose + pytest harness — base of this PR
this	Imgur URLs become configurable
next	the e2e suite that drives Imgur against a WireMock stand-in for api.imgur.com

Based on #1516 rather than master to keep the follow-up harness work off the merge queue. Retarget to master once the parents land.

Why this is a convention, not a new extension point

TransferServiceConfig already exists for exactly this, and WorkerModule already binds a per-extension instance via getForService(ext.getServiceId()). Flickr (FlickrTransferExtension:98), Deezer (DeezerTransferExtension:75) and Synology all consume it; Synology and the generic adapter already read a baseUrl from it. Imgur simply had not adopted the pattern.

Worth noting for anyone weighing this against other adapters: no adapter in the repo had a configurable URL on an export path before this. The two that read URLs from config — GenericTransferExtension and Synology — are import-only (getExporter throws and returns null respectively), which is presumably why they were the ones to need it. Flickr and Deezer consume TransferServiceConfig only for perUserRateLimit.

Imgur is the cheapest place to start, because baseUrl was already a constructor parameter on both ImgurPhotosExporter and ImgurPhotosImporter — only the extension-level constants needed to change. By contrast MicrosoftTransferExtension.BASE_GRAPH_URL is threaded into six importers.

One asymmetry, flagged for review

ImgurTransferExtension takes its config from the injected context.getService(TransferServiceConfig.class), as the convention expects.

ImgurOAuthConfig cannot: an OAuth2ServiceExtension is never handed a service-scoped TransferServiceConfig the way WorkerModule hands one to a TransferExtension. It therefore calls the static TransferServiceConfig.getForService("Imgur") itself, once, at construction.

That means an auth class reads a file named for the transfer side. Both halves genuinely describe the same service's endpoints, and this is the only classpath config mechanism that exists — the alternative was shadowing config/common.yaml, which replaces the jar's copy wholesale rather than merging. Happy to restructure if you'd prefer a separate key or file.

Tests

Adds the first tests for either class — 9 in total, covering defaults, per-key overrides, a config with no serviceConfig section, and a serviceConfig that sets unrelated keys.

There is also a test pinning getServiceName() to "Imgur": PortabilityAuthServiceProviderRegistry does an exact-match lookup on that string, so changing it silently breaks every Imgur job at creation time.

The five pre-existing ImgurPhotoExporterTest cases still pass — verified in a clean worktree containing only this commit, since stacked PRs get no CI in this repo (pull_request: branches: [master] matches the base branch).

docker compose run --rm gradle --no-daemon \
  :extensions:data-transfer:portability-data-transfer-imgur:test \
  :extensions:auth:portability-auth-imgur:test
docker compose run --rm gradle --no-daemon check    # BUILD SUCCESSFUL