Support heterogeneous lists in GeyserHolderSet

[realpeyaj]

Addresses one of #6712 TO-DO list:

• Holder sets now allow mixed references and inline values, e.g. [{dialog:a_dialog, ...}, "geyser:another_dialog"], leading to heterogeneous lists.

Changes

– Added an internal HolderEntry (Direct for inline values, Id for network IDs) in GeyserHolderSet so mixed lists maintain their original sequence.
– Implemented ordered resolution in resolve(session) for mixed entries.
– Made TagCache#is delegate to GeyserHolderSet#contains to avoid IllegalStateException on inline/mixed sets.
– Added unit tests in GeyserHolderSetTest covering all variants (empty, tag, single string, single inline, homogeneous lists, and mixed lists).

Testing

– Ran all 7 test cases in GeyserHolderSetTest (passed).
– Ran the full :core:test suite (passed).
– Tested on Paper 26.3-35-main@dd9d103 with Bedrock player joining and playing cleanly with Floodgate.

Note: This PR used Antigravity (Gemini) to help structure the heterogeneous parsing logic and generate unit tests, of course still accompanied by a human.