fix: validate cached instanceApiVersion before use @W-23157155@

[WillieRuemmele]

Summary

• Validate on read: getCachedApiVersion() now validates the cached instanceApiVersion via validateApiVersion() before returning it. Invalid values (e.g. "latest") are logged at warn level and ignored, forcing a fresh fetch from services/data.
• Validate on write: AuthInfo.update() strips invalid instanceApiVersion values before persisting, preventing the poisoned-cache scenario at the source.
• Improved fallback logging: useLatestApiVersion() now logs the actual version being fallen back to (e.g. v50.0) so the degradation is visible in debug output.

Work Item

@W-23157155@: getCachedApiVersion() returns cached instanceApiVersion without re-validating, silently degrading to jsforce default (v50)

Proof of Work

• Tests: 1158 passing (unit), 57 passing (NUT)
• Lint: clean (0 errors, pre-existing warnings only)
• Type check: clean
• Build: clean

Test plan

• Set instanceApiVersion: "latest" in ~/.sfdx/<username>.json and verify connection.getApiVersion() returns the org's real max version (not 50.0)
• Verify that AuthInfo.save({ instanceApiVersion: 'latest' }) strips the invalid value and logs a warning
• Verify valid cached versions (e.g. "61.0") are still used from cache without a network call
• Verify expired cache (>24h) still triggers a re-fetch as before