feat: generate and push fallback config on update failure

[czeslavo]

What this PR does / why we need it:

Makes KongClient try to recover from regular config update failures by generating a fallback configuration, excluding affected objects reported back by gateways.

Enables integration tests with the FallbackConfiguration feature gate enabled to ensure no existing scenarios are broken.

Adds only unit tests covering KongClient. More integration/E2E test coverage is expected to be added under #5933.

Which issue this PR fixes:

Closes #5931.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 71.2%. Comparing base (763d7d1) to head (f877388).
Report is 5 commits behind head on main.

❗ Current head f877388 differs from pull request most recent head f62fef2

Please upload reports for the commit f62fef2 to get more accurate results.

Files	Patch %	Lines
internal/dataplane/kong_client.go	84.8%	5 Missing and 5 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6071     +/-   ##
=======================================
- Coverage   74.6%   71.2%   -3.4%     
=======================================
  Files        194     195      +1     
  Lines      18861   18956     +95     
=======================================
- Hits       14072   13513    -559     
- Misses      3814    4476    +662     
+ Partials     975     967      -8     

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