fix(operator): rename BackendAuthType enum value to camelCase

[tmchow]

The enum value `external_auth_config_ref` was the only snake_case type discriminator in the CRD codebase. Every other value uses camelCase (`tokenExchange`, `headerInjection`, `bearerToken`, `embeddedAuthServer`, `awsSts`, etc.). Renamed to `externalAuthConfigRef` for consistency before the API moves past v1alpha1.

Changes:

• Renamed the constant string value in `virtualmcpserver_types.go`
• Updated kubebuilder validation Enum markers and CEL validation rules
• Updated all test fixtures across 5 test files
• Regenerated CRD manifests and API docs

Build, lint (0 issues), and compilation all pass.

Fixes #4542

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.73%. Comparing base (d851c69) to head (2571582).

Files with missing lines	Patch %	Lines
cmd/thv-operator/pkg/vmcpconfig/converter.go	22.22%	6 Missing and 1 partial ⚠️
...hv-operator/api/v1alpha1/virtualmcpserver_types.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4563      +/-   ##
==========================================
+ Coverage   68.71%   68.73%   +0.02%     
==========================================
  Files         517      517              
  Lines       54817    54824       +7     
==========================================
+ Hits        37666    37686      +20     
+ Misses      14252    14241      -11     
+ Partials     2899     2897       -2     

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[tmchow]

Ran controller-gen for both operator-generate and operator-manifests -- no changes produced, so the generated files are already in sync.

The deprecation warning is in converter.go and fires when the old external_auth_config_ref value is used. Using log.Info since controller-runtime's logr doesn't have a Warn level, but happy to switch to a different approach if you prefer.