fix(streamable): emit explicit SSE event name for JSON-RPC responses

[syf2211]

Summary

The streamable-http proxy now writes event: message before each JSON-RPC SSE data: line, matching reference MCP server transports and ToolHive's own SSE transport.

Motivation

Clients such as @ai-sdk/mcp only dispatch SSE frames where event === "message" and silently drop event-less frames. When stdio MCP servers are fronted by the streamable proxy, initialize hangs forever because the proxy emitted data:-only frames.

Fixes #5655

Changes

• Add sseMessageEvent constant ("message") in streamable_proxy.go
• Update both success and error SSE write paths in handleSingleRequestSSE to emit event: message\ndata: ...\n\n
• Add TestSSEResponseIncludesEventName regression test covering POST initialize with Accept: text/event-stream

Tests

go test ./pkg/transport/proxy/streamable/... -count=1

All tests pass (including the new SSE event name test).

Notes

• This is defense-in-depth alongside the client-side fix tracked in [Bug] Playground fails to detect tools for stdio-based MCP servers toolhive-studio#2333
• Batch POST + Accept: text/event-stream still returns JSON (pre-existing behavior, out of scope)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.35%. Comparing base (7c574e2) to head (383de73).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/transport/proxy/streamable/streamable_proxy.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5657      +/-   ##
==========================================
- Coverage   70.37%   70.35%   -0.02%     
==========================================
  Files         649      649              
  Lines       66170    66185      +15     
==========================================
- Hits        46568    46566       -2     
- Misses      16231    16265      +34     
+ Partials     3371     3354      -17     

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