fix(protocol): send the original McpError message, not the prefixed one

[fszcd]

Root cause

McpError's constructor builds .message as MCP error <code>: <message>. When a v1 request handler throws, the protocol layer serialized error.message — prefix included — onto the wire. A v1 peer then reconstructs with new McpError(code, wireMessage), adding the prefix a second time:

client received: MCP error -32601: MCP error -32601: Unknown tool: nope

Per @pj-workspace's analysis on the issue, the wire convention is an unprefixed message: v2 servers already emit it (v2 removed the constructor prefix in #1727), and both v1 and v2 clients add the prefix when reconstructing. Only the v1 server leaks its local prefixed form. Since changing McpError.message in v1 would be breaking (callers may match on it), this PR takes the additive route.

Fix

• McpError retains the constructor argument as a new readonly originalMessage (.message is unchanged — no breaking change).
• The protocol's error serialization sends originalMessage when the thrown error is an McpError, so the wire carries exactly what the handler author wrote. Other error types keep the existing error.message ?? 'Internal error' behavior.

Effect on @pj-workspace's matrix: the v1→v1 row goes to 0 wire prefixes / 1 client prefix, matching v2→v2; v1→v2 stays clean.

Tests

New test/issues/test_2786_mcp_error_wire_message.test.ts runs a real Client↔Server pair over InMemoryTransport, captures the raw JSON-RPC error at the transport boundary, and asserts:

• wire error.message is Unknown tool: nope (no prefix)
• the client-visible error is MCP error -32601: Unknown tool: nope (exactly one prefix) with the right code and originalMessage

Fails on unpatched v1.x (wire message has the prefix), passes with the fix. Full suite: 53 files / 1648 tests pass; the 2 test/client/stdio.test.ts pipe errors reproduce identically on an unmodified checkout (Windows-local, unrelated). tsgo --noEmit and eslint clean. (prettier --check . flags 227 untouched files on this Windows checkout due to autocrlf CRLF; the three changed files pass individually and are committed as LF.)

Fixes #2786

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commit: 95a404d

[koriyoshi2041]

Verified at 95a404d7: v1 keeps the existing local McpError.message API while serializing the constructor-supplied message once at the wire boundary, and non-McpError serialization/data handling stays unchanged. The focused real Client↔Server transport regression passes locally (1/1), as does npm run typecheck; hosted build, unit, E2E, and conformance checks are green. I found no correctness blocker.

[fszcd]

Closing this for now as the contribution workflow is being consolidated. If the change remains useful, it can be reopened or resubmitted later. Thank you for the review time.