fix(server): surface parse/validation transport errors via onerror in streamable HTTP

[Maverick-666]

Summary

This PR fixes missing onerror propagation in StreamableHTTPServerTransport when request parsing/validation fails.

What changed

• Updated packages/server/src/server/streamableHttp.ts:
  • call this.onerror?.(...) when req.json() fails
  • call this.onerror?.(...) when JSONRPCMessageSchema.parse(...) fails
• Added regression tests in packages/server/test/server/streamableHttp.test.ts to verify both paths trigger onerror

Why

Previously, these transport-level errors could be handled in HTTP responses but were not reported through onerror, making observability and custom error handling inconsistent.

Validation

• pnpm --filter @modelcontextprotocol/server test -- test/server/streamableHttp.test.ts
• Targeted lint/format checks for modified files

Closes #1395

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commit: 02634bf

[Maverick-666]

Quick follow-up: all required project CI checks are passing, and this PR is ready for code-owner review.

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