controller: add consistency check for gateway object namespaces

[nacx]

Description

The previous gateway controller code made assumptions about the namespace consistency across gateway resources. This change adds an explicit check and returns an error if there are inconsistencies.

Related Issues/PRs (if applicable)

N/A

Special notes for reviewers (if applicable)

N/A

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.70%. Comparing base (6214f8b) to head (77d52f4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/gateway.go	72.72%	6 Missing and 3 partials ⚠️

❌ Your patch status has failed because the patch coverage (72.72%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2274   +/-   ##
=======================================
  Coverage   84.70%   84.70%           
=======================================
  Files         144      144           
  Lines       21206    21214    +8     
=======================================
+ Hits        17962    17970    +8     
  Misses       2162     2162           
  Partials     1082     1082           

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