fix: pass logger instances via loggerInstance (fastify v5 rejects instances in logger)

[kilisamemarisaaa]

Summary

fastify v5 only accepts a configuration object in the logger option and throws FST_ERR_LOG_INVALID_LOGGER_CONFIG when given an instance — instances must go through loggerInstance. fastify-cli converges user-provided logger instances — from --logging-module (#775), --options files (the setup described in #574), or server options — into options.logger, so passing a real Pino instance crashes fastify start at startup:

FastifyError [Error]: logger options only accepts a configuration object.
    code: 'FST_ERR_LOG_INVALID_LOGGER_CONFIG'

This was invisible to the suite because the only logger tests use a config-object fixture (test/data/custom-logger.js = { name, customLevels: {...} }); the instance path had zero coverage.

Fix: after all option merges converge in start.js, route any logger value exposing child() to loggerInstance (4 lines). Config-object loggers (including the existing fixtures and the { level } default) are untouched.

Regression test: -L with a fixture exporting a real Pino instance; asserts the logger level survives into fastify.log.

Verification

• New test passes; full test/**/*.test.js suite unchanged (78/85 locally on Windows — the 5 failures are the Windows-only EBUSY family documented in fix: restore CI test execution and repair CJS/CLI regressions from dependency bumps #910)
• npm run lint clean

Depends on #910 — this branch contains it (the start.js path cannot even run without those interop fixes; rebase after that merges if needed).

[Tony133]

There are the same commits as in this PR: #910 Can you create a clean PR with the changes described? Or do a rebase?

[kilisamemarisaaa]

Thanks for the review @Tony133 — you're right, the branch was cut from the #910
branch and carried its commits. I've rebuilt the branch on top of the latest
upstream/main so it now contains only the logger-instance change (single
commit); CI is rerunning on the clean history.