[#24316] Reduced InstanceCache key warning spam

[Danipiza]

Description

This PR fixes repeated FASTDDSSPY_INSTANCECACHE warnings caused by missing key metadata on incoming DynamicTypes, while preserving current key-caching behavior.

Changes

• Added early short-circuit in InstanceCache::add_or_update_instance:
  • If no key fields are discovered for a topic, skip key serialization.
• Added one-time warning tracking set in InstanceCache.
• Improved key field cache behavior:
  • Do not permanently lock an empty key-field cache.
  • Recompute key fields when cached list is empty, allowing recovery if keyed schema arrives later.

Related PR Display the list of keys in a topic

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.73%. Comparing base (b85532f) to head (0c503bb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...dsspy_participants/src/cpp/model/InstanceCache.cpp	50.00%	0 Missing and 4 partials ⚠️

❗ There is a different number of reports uploaded between BASE (b85532f) and HEAD (0c503bb). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (b85532f)	HEAD (0c503bb)
	11	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
- Coverage   62.27%   56.73%   -5.54%     
==========================================
  Files          29       28       -1     
  Lines        2502     1960     -542     
  Branches     1070      772     -298     
==========================================
- Hits         1558     1112     -446     
+ Misses        382      367      -15     
+ Partials      562      481      -81     

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[raulojeda22]

LGTM