fix(relay): deliver media from Git Bash paths and legacy connectors on Windows

[AutomatteAsia]

Problem

On native Windows, Hermes cannot deliver files (e.g. .pdf) to messaging platforms. The bundled Git Bash (MinGit) terminal reports paths in Unix format (/c/Users/...), and two code paths drop or reject the media:

1. Git Bash paths fail the delivery gate. validate_media_delivery_path() checks Path.is_absolute(), which returns False for /c/Users/... (no drive letter). Every file the agent names is rejected as "unsafe" and silently falls back to a text notice. The same rejection hits _normalize_media_tag_path() (MEDIA: tags) and extract_local_files() (bare paths in the reply).

2. Relay connectors never advertise send_media. send_media was added after the supported_ops discovery field (Phase 1 feat(relay): Phase 1 parity — supported_ops discovery, wire identity fields, /handoff aliasing, provision displayName #71300 came before Phase 2 feat(relay): Phase 2 media parity — send_media egress + inbound media localization #71363). A legacy connector (empty supported_ops) falls back to LEGACY_OPS = ("send", "edit", "typing", "follow_up"), which omits send_media - so the op-gated media lane is never attempted, even when the connector implements it.

Fix

1. Normalize Git Bash paths in the three path consumers - validate_media_delivery_path, _normalize_media_tag_path, extract_local_files. On os.name == "nt", convert /c/Users/... ? C:\Users\.... The conversion is guarded on the drive letter actually existing (os.path.isdir("C:\\")), so genuine POSIX-style paths like /r/a.png are left untouched.

2. Fail open for send_media on legacy connectors. Respect an explicit op declaration that excludes send_media (the caller falls back to text, unchanged), but for legacy connectors (empty supported_ops - which predate the discovery field and can therefore never advertise the op even when they implement it), attempt the lane and let a transport decline degrade to the same text fallback.

Verification

• Reproduced on current main: Git Bash path rejected by validate_media_delivery_path; relay send_media gated out for empty supported_ops.
• After fix: /c/Users/... validates, extracts via MEDIA: tag and bare-path detection, and resolves to the real file; native C:\... paths behave unchanged.
• Test suites: tests/gateway/relay/test_relay_media.py (12/12), test_media_tag_formatting_variants.py, test_media_tag_cleanup.py, test_media_extraction.py all green. The 6 remaining failures in the media suite are pre-existing Windows-vs-POSIX path-format assertions that also fail on clean main without this change.