docs : added changed-file fallback examples to CI test-selection guide

[tmdeveloper007]

Closes #1101.

Summary of What Has Been Done:
Added a "Fallback Behavior" section to docs/ci-test-selection.md documenting how the CI test selection handles edge cases where changed-file detection fails or returns incomplete results.

Changes Made:

• Added three fallback scenarios: detect_changes.py failure, empty file list, and Git history unavailability (shallow clone)
• Added concrete bash examples showing the fallback output for each scenario
• Updated the "Why This is Safe" conservative fallbacks list to include the three new cases

Impact it Made:

• Reduces confusion for contributors running CI in non-standard environments (shallow clones, S3 archives)
• Documents the safety guarantees that prevent silent test skipping in degraded environments
• Makes the fallback behavior explicit and verifiable

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.

[utksh1]

Focused change with green checks; approving.