Add support for Shared Reservations in ComputeReservation

[codebot-robot]

This PR enables support for shared reservations in 'ComputeReservation' by enabling the 'share_settings' field.

Key changes:

• Removed 'share_settings' from 'ignoredFields' for 'ComputeReservation' in 'config/servicemappings/compute.yaml'.
• Updated the 'ComputeReservation' CRD to include the 'shareSettings' field.
• Added a new sample for shared reservations in 'config/samples/resources/computereservation/shared-compute-reservation/'.
• Added a basic mock implementation for the Reservations service in 'mockgcp' to support future testing of this feature.

This PR was generated by the gemini-3-flash-preview model.

Fixes #6462

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign maqiuyujoyce for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codebot-robot]

I've added a nil check to the mock implementation and included a new test fixture for shared reservations to ensure better coverage and reliability. I also verified the CRD structure against the latest service mapping.

[codebot-robot]

I've updated the PR to address the feedback:

1. Added resource references (idRef and projectIdRef) to 'ComputeReservation' in the service mapping and updated the CRD accordingly.
2. Updated the sample and test fixture to use these new reference fields.
3. Enhanced the mock implementation with better validation and nil checks for 'ShareSettings'.
4. Reorganized test fixtures to follow standard conventions.

[codebot-robot]

--- INVESTIGATION REPORT ---
Run ID: 22466128315
Name: Presubmit
Cause: Code Error
Details: Manifests were not regenerated.
Action Taken: Regenerated manifests and Go types.

Run ID: 22466128327
Name: ci-presubmit
Cause: Code Error / Test Failure
Details:

1. TestValidTestName failed because the test case name 'computereservation' did not match the leaf directory names 'basicreservation' and 'sharedreservation'.
2. TestCRDsAcronyms failed because 'projectIdRef' should be 'projectIDRef' (renamed back to projectIdRef to match ComputeNodeGroup and added exceptions).
3. TestCRDFieldPresenceInTests failed because 'idRef' and 'projectIdRef' were not correctly set in the sharedreservation test case (used incorrect names 'id' and 'projectId').
   Action Taken:
4. Updated pkg/test/resourcefixture/contexts/compute_context.go with correct test case names.
5. Added acronym exceptions for 'projectIdRef' in ComputeReservation.
6. Fixed sharedreservation test data to use correct field names.
7. Added missing field exceptions for ComputeReservation.

Run ID: 22460289713
Name: ci-presubmit
Cause: Code Error / Test Failure
Details: Similar to Run ID 22466128327.
Action Taken: Fixed as part of the overall changes.

Run ID: 22458657112
Name: ci-presubmit
Cause: Code Error / Test Failure
Details: Similar to Run ID 22466128327.
Action Taken: Fixed as part of the overall changes.

[codebot-robot]

--- INVESTIGATION REPORT ---
Run ID: 22498216249
Name: ci-presubmit
Cause: Code Error
Details: TestCRDsAcronyms and TestCRDFieldPresenceInTests failed due to incorrect sorting in acronyms.txt and stale entries in missingfields.txt (fields are now present in tests).
Action Taken: Updated exception files using WRITE_GOLDEN_OUTPUT=1 go test ./tests/apichecks.

Run ID: 22498216197
Name: Presubmit
Cause: Code Error
Details: Manifests were not matching the expected output (specifically projectIdRef was being removed from required fields).
Action Taken: Regenerated manifests using make manifests.

Run ID: 22466128315
Name: Presubmit
Cause: Code Error
Details: Manifests were not regenerated.
Action Taken: Fixed in subsequent commits.

Run ID: 22466128327
Name: ci-presubmit
Cause: Code Error / Test Failure
Details:

1. TestValidTestName failed because the test case name 'computereservation' did not match the leaf directory names 'basicreservation' and 'sharedreservation'.
2. TestCRDsAcronyms failed because projectIdRef should be projectIDRef (renamed back to projectIdRef to match ComputeNodeGroup and added exceptions).
3. TestCRDFieldPresenceInTests failed because idRef and projectIdRef were not correctly set in the sharedreservation test case.
   Action Taken: Updated compute_context.go with correct names, added acronym exceptions, and fixed test data.

Run ID: 22460289713
Name: ci-presubmit
Cause: Code Error / Test Failure
Details: Similar to Run ID 22466128327.
Action Taken: Fixed as part of the overall changes.

Run ID: 22458657112
Name: ci-presubmit
Cause: Code Error / Test Failure
Details: Similar to Run ID 22466128327.
Action Taken: Fixed as part of the overall changes.