Restore logger state after each fshToFhir call

[cpruijsen]

Description:

fshToFhir saves both silent and level on the process-wide winston logger, applies options.logLevel, then restores them in finally, including on the invalid-logLevel return and when processing throws. After logLevel: 'silent', logger.transports[0].silent stayed true, so a later call with logLevel: 'error' (or with no option) still wrote nothing. After logLevel: 'error', logger.level stayed 'error' for later calls that omitted the option. Errors and warnings in the returned object were already correct; only console logging leaked.

'silent' is not a winston level. It sets the Console transport's silent flag, so restoring logger.level alone is not enough.

The reporter's sequence in #1645 is the spec:

await fshToFhir('', { logLevel: 'silent' });
await fshToFhir('', { logLevel: 'error' });
await fshToFhir('');

Restore both fields in try/finally rather than resetting to info / not-silent at the start of each call and leaving the last call's settings on the logger (that restore stops a silent call from muting every other logger user in the process, and matches the existing save/restore pattern in withDebugLogging and restoreMainLogger). Reset-at-start would also make sequential fshToFhir calls independent, but it would leave the singleton muted (or at error) after return. Happy to switch to reset-at-start if you want logger.level to remain 'error' after a call.

Tests for during-call application use the existing loadExternalDependencies spy (it runs after logLevel is applied). New cases live in a #LoggerIsolation describe so they do not collide with the #EndToEnd work on #1642.

Most of the src/run/FshToFhir.ts diff is re-indentation inside the new try; hiding whitespace shows the actual change.

Testing Instructions:

• npm test -- test/run/FshToFhir.test.ts
• In a Node REPL or script that imports fshToFhir, run the three-call sequence from fshToFhir does not reset the logger between calls #1645 and confirm the second call is not silent and the third is back at the default info level.
• Ran here: npm test -- test/run/FshToFhir.test.ts (16 passed; 5 of them fail on master), tsc --noEmit, and prettier --check / eslint on the two changed files.

Related Issue:

Fixes #1645

[cmoesel]

Thank you @cpruijsen for this PR. I apologize for the delay in reviewing it. I had a couple of large and complicated PRs that I had to get through first. I just merged one of them (#1637) today and I'm working my way through #1641 next. I hope to review the remaining PRs soon after that.