test(mcp-server): complete the unit test matrix (prompt 20)

[gilgardosh]

Summary

Implements Prompt 20 — Unit test matrix completion from
docs/mcp/implementation-blueprint.md.
Rounds out deterministic, service-free unit coverage across the core modules the blueprint calls
out. Stacked on top of the prompt 19 branch (claude/mcp-prompt-19-metrics-tracing).

What's added

• Env config validation — cover loadEnv's fail-fast startup path (invalid environment → printed
  report + process.exit(1), with process.exit/console.error stubbed) and the valid-source happy
  path. dotenv is pointed at a nonexistent file so no real .env leaks in.
• Token validation edge cases — verification through a key-resolver function, rejection of a
  signature-valid token that is missing the sub claim, and the per-request principal store
  (setAuthPrincipal/getAuthPrincipal round-trip + isolation between requests).
• Registry/schema validation — charges tool now covers a single impossible toDate and an
  over-cap date range; the balance report tool covers a regex-passing but impossible date.
• Output truncation — default-summary branches (No results., and returned-of-total with and
  without the truncated marker).

The policy evaluator and error mapper (taxonomy) already had thorough dedicated suites; the gaps
addressed here are the ones the coverage report flagged.

Coverage

Package branch coverage improves from ~83.9% → ~85.9% (statements ~89.5% → ~90.8%); token.ts
reaches full line coverage. All tests are deterministic and call no external services.

yarn workspace @accounter/mcp-server lint, typecheck, and test (236 passing) all green.

🤖 Generated with Claude Code

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[gemini-code-assist]

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[Copilot]

Pull request overview

This PR completes the unit test matrix for @accounter/mcp-server core modules, adding deterministic, service-free coverage for env loading/validation, auth token verification edge-cases, tool date-range guards, and list output summaries/truncation behavior.

Changes:

• Add loadEnv fail-fast startup tests (invalid env triggers report + process.exit(1), valid env returns config without exiting).
• Expand auth token tests for key-resolver verification, missing sub claim rejection, and per-request principal storage isolation.
• Extend tool-level date validation tests (impossible-but-regex-valid dates, oversized ranges) and output summary string coverage.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/mcp-server/src/config/tests/env.test.ts	Adds unit coverage for loadEnv exit-on-invalid and success-with-valid-source paths without leaking real .env.
packages/mcp-server/src/auth/tests/token.test.ts	Adds token verification edge-case tests and request-scoped principal store round-trip/isolation coverage.
packages/mcp-server/src/tools/tests/charges.test.ts	Adds date parsing/range-cap validation tests for searchChargesTool (invalid toDate, over-cap ranges).
packages/mcp-server/src/tools/tests/reports.test.ts	Adds impossible-date test for balanceReportTool to ensure handler-level Date.parse guard rejects it.
packages/mcp-server/src/tools/tests/output.test.ts	Adds coverage for shapeListResult default summary branches including truncated marker variants.

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