Delete SSE compat path (MCP_TRANSPORT=sse) — migration-window cleanup

[romer8]

Context

PR #111 (merged 2026-05-08) migrated the in-app TethysDash MCP server from legacy SSE at `/sse` to Streamable HTTP at `/mcp` by default. The legacy path remains reachable behind `MCP_TRANSPORT=sse` for users with `/sse`-suffixed localStorage configs to migrate.

The plan (2026-05-08-001) committed to a concrete deletion trigger so this doesn't rot into permanent dead code.

Deletion trigger

Delete the SSE compat path when either is true (whichever comes first):

• (a) Team confirms no chatbox-core localStorage entries reference `/sse` in active workspace usage
• (b) 4 weeks have elapsed since PR feat(mcp): /sse → /mcp Streamable HTTP migration + env-driven CORS + loopback default (PR 2/2) #111 merged → 2026-06-05

What to delete

• `tethysapp/tethysdash/mcp/tethysdash_mcp_server.py`:
  • `_patch_sse_transport_for_cors` function and its conditional invocation in `main`
  • The `if transport == "sse"` branch in the entrypoint
  • The unused `StarletteResponse` import (only the SSE patch uses it)
• `tethysapp/tethysdash/tests/mcp/test_transport_and_cors.py`:
  • The three R12 SSE-patch regression tests (`test_sse_patch_*`)
• `CHANGELOG.md`: add an Unreleased entry noting the SSE compat path is removed
• `CLAUDE.md`: drop the `MCP_TRANSPORT=sse` mention from the server description

Verification

• 543 / 543 existing MCP contract tests still pass (SSE-specific tests removed; no other test depends on the SSE path)
• Manual smoke: server still starts and serves `/mcp`; `MCP_TRANSPORT=sse` no longer has an effect (env var becomes a no-op)

Plan reference

`docs/plans/2026-05-08-001-fix-mcp-validation-and-streamable-http-migration-plan.md` → Documentation/Operational Notes → "SSE compat path deletion trigger"