Handle undefined bridge config content in tests

[willwashburn]

Summary

• make skip non-string and empty file content before JSON parsing
• prevent noisy parse errors when mocked fs returns undefined in tests
• add a regression test to ensure non-string bridge config content is ignored without logging parse errors

Validation

• ran

@agent-relay/config@2.3.14 test
vitest run

RUN v3.2.4 /Users/will/Projects/relay/packages/config

✓ src/relay-config.test.ts (4 tests) 2ms
✓ src/bridge-config.test.ts (11 tests) 5ms
✓ src/agent-config.test.ts (16 tests) 16ms
✓ src/schemas.test.ts (7 tests) 3ms
✓ src/relay-file-writer.test.ts (27 tests) 32ms

Test Files 5 passed (5)
Tests 65 passed (65)
Start at 08:58:29
Duration 299ms (transform 172ms, setup 0ms, collect 287ms, tests 58ms, environment 0ms, prepare 294ms)

• all tests pass (65/65)

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.