Allowing disabling logging and configuring optional fields in LB backend services (#3940)

* fix(modules): allow disabling logging and configuring optional fields in LB backend services

Replaced 'log_sample_rate' (number) with 'log_config' (object) in all Load Balancer Backend Service modules. This allows explicitly disabling logging ('enable = false') and configuring advanced options like 'optional_mode' and 'optional_fields', resolving infinite plan drift and the inability to disable logging.

Affected modules:
- net-lb-app-ext-regional
- net-lb-app-ext
- net-lb-app-int-cross-region
- net-lb-app-int
- net-lb-ext
- net-lb-int
- net-lb-proxy-int

Added test cases and updated documentation.

Fixes #3914

* style: format variables files with terraform fmt

* docs: add critical linting rule for AI agents to GEMINI.md

GEMINI.md

@@ -270,5 +270,6 @@ Run the specific `pytest` plan test. The test will fail, and the captured output
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