test(mcp): add contract tests for buildApiErrorMessage helper

[JayDS22]

The clean test-only follow-up to #155 (which merged Jun 13). Adds eight contract tests for the buildApiErrorMessage helper, plus a one-line export on the helper itself so the tests can import it rather than re-declare it.

This is the PR that was discussed as the replacement for the now-closed #153. The original #153 bundled the helper + error-path rewrites + the test file together; #155 took the helper and the rewrites, this PR takes the tests cleanly on top.

What the tests cover

Two describe blocks, eight cases:

buildApiErrorMessage itself:

• HTTP 404, 400, 500, 422 status codes surface in the returned message
• Resource identifiers ('late-delivery', 'agreement-abc-123', 'agr-1', format names like 'pdf') are preserved in the error context
• Body-read failures (stream already consumed, network error mid-read) degrade gracefully to "No error details available" instead of throwing

Error distinctness (the original motivation):

• A 404, 400, and 500 produce three distinct messages so MCP clients can tell them apart
• Every message starts with the context string passed by the caller

Why the export change

The helper was added to mcp.ts as a module-local function in #155. This PR adds export to its declaration (one line) so mcp.test.ts can import it directly rather than re-declaring its logic, which was the coordination concern Niall and I discussed when planning to split #153.

Diff

server/handlers/mcp.ts      |   2 +-      (export keyword)
server/handlers/mcp.test.ts | 118 +++++   (new test file)

Coordination notes

• Doesn't conflict with @Satvik77777's test(mcp): add test suite for mcp.ts handler and fix session header bug #189 (his test file uses InMemoryTransport for tool-registration tests; mine covers the helper). If test(mcp): add test suite for mcp.ts handler and fix session header bug #189 merges first, this PR will simply have a more populated mcp.test.ts to coexist with; happy to rebase.
• Full server suite: 61/61 passing.

Closes #152 (jointly with merged #155).

[github-actions]

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