mcp: answer a request sent before initialize with Invalid Request

[talhayme]

A legacy server session that has not been initialized refuses a request other than initialize or ping. The refusal was a plain error, which reaches the wire as JSON-RPC error code 0. It is now -32600 (Invalid Request).

This is a behavioral change, so MCPGODEBUG=nopreinitinvalidrequest=1 restores the previous code. The option is documented in docs/mcpgodebug.md under 1.9.0, and TestServerHandle_LegacyCallBeforeInitialize checks both settings.

Fixes #1367

AI disclosure: written with Claude Code; I reviewed the change and ran go test ./... and the touched tests under -race.

🤖 Generated with Claude Code

[guglielmo-san]

@talhayme thank you for the contribution!

[Goldziher]

Confirmed downstream. Using the reproduction from #1367 (tools/list sent before initialize over IOTransport):

• v1.8.0: {"error":{"code":0,"message":"method \"tools/list\" is invalid during session initialization"}}
• b5d319b (this PR merged): same message, "code":-32600

We were rewriting the code to -32600 in a transport wrapper (Goldziher/ai-rulez#385) and will drop that for the authoring server once a release includes this. Thanks for the quick fix.

[talhayme]

Thanks for confirming. The other places where a plain error reaches the wire as code 0 (prompt, resource and raw tool handlers, middleware) are collected in #1394, in case your wrapper was covering those too.

AI disclosure: drafted with Claude Code; I reviewed it.