fix(health-check): Return 503 during shutdown to prevent ECONNREFUSED

[AliRaZa1121]

• Add opt-in 'failReadinessOnShutdown' (default false)
• Classify fulfilled results with {status:'down'} as errors
• Capture sync throws from indicators via async wrapping
• Status precedence: (shutdown+flag) -> 'error'; else errors -> 'error'; else shutdown -> 'shutting_down'; else 'ok'
• Tests: shutdown behavior (flag on/off), legacy sync-throw, 'down' result classification

Fixes #2298

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/terminus/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information