feat(gateway): declare per-adapter media-send capabilities

[sprmn24]

What does this PR do?

Adds a media_capabilities field to PlatformEntry and the relay CapabilityDescriptor, so each platform adapter declares which media kinds (voice/video/document/image_file/animation) it natively sends via a real send_* override, as opposed to falling back to BasePlatformAdapter's generic text-notice fallback.

This is groundwork for the plugin-interface expansion discussed in the #Developer channel: once tools/send_message_tool.py's cross-channel send routes through this declared capability set instead of its current hardcoded per-platform allowlist, adapters (core or plugin) will be able to add/fix media support entirely inside their own register(ctx) call — no core send_message_tool.py edit required.

Related Issue

Groundwork toward #17261 (Slack cross-channel media falls through to text-only). This PR is additive metadata only and does not change send_message_tool.py's routing yet — that follow-up is intentionally separated out pending direction on the capability-routing approach.

Type of Change

• New feature (non-breaking change which adds functionality)

Changes Made

• gateway/platform_registry.py: new media_capabilities: frozenset[str] = frozenset() field on PlatformEntry.
• gateway/relay/descriptor.py: new media_capabilities: tuple[str, ...] = () field on CapabilityDescriptor, projected in from_platform_entry(), with a JSON round-trip fix in from_json() (list→tuple cast).
• docs/relay-connector-contract.md: documented the new descriptor field (required by test_contract_doc_conformance.py).
• All 15 platform adapters: added media_capabilities=frozenset({...}) to their register(ctx) call, matching their actual send_voice/send_video/send_document/send_image_file/send_animation overrides (verified per-adapter against source, not assumed).

How to Test

scripts/run_tests.sh tests/gateway tests/plugins — full suite passes except two pre-existing flakes unrelated to this change (confirmed identical failures on unmodified main): an mtime-memoization test and an asyncio-timeout race in test_startup_restart_race.py.

Checklist

• Single focused commit
• Tests pass (see above)

[alt-glitch]

This was generated by AI during triage.

Related to #36817 (per-adapter MEDIA_KINDS capability, fail-closed routing) — both concern declaring which media kinds each adapter natively sends. This PR is additive metadata only (media_capabilities on PlatformEntry / CapabilityDescriptor + all 15 adapters) and groundwork toward #17261 (Slack cross-channel media falls through to text); it does not yet change send_message_tool.py routing. Flagging the overlap so a reviewer can decide which capability-declaration approach to converge on.

[teknium1]

Thanks for isolating the capability declaration work.

Problems

• This does not yet change the reported Slack behavior: current tools/send_message_tool.py:1034-1047 omits Slack from MEDIA delivery, and its Slack branch calls the text-only standalone sender (tools/send_message_tool.py:1051-1062; plugins/platforms/slack/adapter.py:4350-4355).
• CapabilityDescriptor.from_platform_entry() has no production caller on current main; its only call sites are descriptor tests. RelayAdapter._apply_descriptor() currently consumes only length and markdown behavior (gateway/relay/adapter.py:221-225). Adding this field to the connector contract therefore needs agreement on the #36817 capability model before it becomes useful.
• The diff adds 15 manually duplicated declarations without coverage that they match concrete adapter send_* overrides or that a non-empty descriptor field round-trips through JSON.

Suggested changes

• Converge with #36817 on whether this belongs in the relay descriptor, then add projection/JSON and declaration-to-override contract tests.
• Keep the Slack routing/upload work as the explicit follow-up; this metadata alone cannot fix #17261.

Automated hermes-sweeper review.

[teknium1]

This extends the connector handshake contract, but current production has no caller of from_platform_entry() and RelayAdapter._apply_descriptor() does not consume this capability. Please first converge with #36817 on the negotiated capability model, or keep this metadata out of the relay descriptor until its connector consumer ships.

[teknium1]

Please add a contract test that registered media kinds correspond to concrete send_* overrides. The declaration is manually duplicated metadata, and the current test suite does not cover these new registrations or a non-empty descriptor JSON round trip.

[sprmn24]

Reviewed #36817 , the MEDIA_KINDS class-attribute approach on BasePlatformAdapter and this PR's PlatformEntry registration-time field serve the same goal from different angles. Happy to drop media_capabilities from the relay CapabilityDescriptor (the premature-contract concern) and keep only the PlatformEntry field as a lighter complement to #36817's dispatch-side work, if that's the preferred split. Or I can rebase on top of #36817 once it lands. What's the direction you want?