test: add unit tests for resolveApiKey precedence in config.zig

[javimosch]

What changed

Added 8 unit tests to src/config.zig for the resolveApiKey function, which implements the 6-level API key precedence chain used across every provider invocation.

Previously this function had zero test coverage despite being a critical path — a regression in any precedence level would silently break auth without any failing test.

Test cases

Test	Precedence level covered
explicit --api-key wins all lower sources	Level 1
config keys[provider] map beats provider env var	Level 2
provider primary env var is used when above absent	Level 3
second env key tried when first absent (xiaomi: PIZIG_API_KEY)	Level 3b
config_api_key beats TAU_API_KEY	Level 4
TAU_API_KEY used as last resort before null	Level 5
returns null when no key available anywhere	Fall-through
empty-string env values are skipped in precedence	Guard check

Verification

zig build test --summary all
# Build Summary: 3/3 steps succeeded; 141/141 tests passed (was 133)

Closes #56 (mago task #56)

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 256f0403-874a-4bbd-a031-0b0fceb76bfb

📥 Commits

Reviewing files that changed from the base of the PR and between 3e3b3b7 and d6df262.

📒 Files selected for processing (1)

• src/config.zig

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch mago/task-56

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[javimosch]

Review — Head of Org Engineering: The diff adds only test cases for the existing resolveApiKey function in src/config.zig, covering all documented priority levels (explicit key, keys map, provider env var, config_api_key, TAU_API_KEY, null). The tests are syntactically valid Zig, scoped to the correct file, and introduce no correctness bugs, security issues, or secrets.

(approved — review-only mode; merge when ready.)