[4.0.3 backport] CBG-5118: Support request-provided oldDoc values in Sync Function Dry-Run

[bbrks]

Clean cherry-pick of #8019 for 4.0.3

Integration Tests

• n/a

[github-actions]

Redocly previews

• Admin API
• Public API
• Metric API
• Diagnostic API

[Copilot]

Pull request overview

This PR backports support for request-provided oldDoc values in the Sync Function Dry-Run API to version 4.0.3. The change allows users to specify an inline oldDoc body when testing sync functions, expanding the API's flexibility beyond the existing behavior of fetching oldDoc from the bucket via doc_id.

Changes:

• Adds support for inline oldDoc parameter in the dry-run API request payload
• Updates API documentation to reflect the new oldDoc parameter and valid parameter combinations
• Adds validation to prevent conflicting usage of doc_id with inline oldDoc

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
rest/diagnostic_doc_api.go	Implements oldDoc support in the dry-run handler, adds validation logic, and renames function/constant for consistency
rest/diagnostic_doc_api_test.go	Adds test coverage for inline oldDoc usage and updates existing tests to use renamed constant
docs/api/paths/diagnostic/keyspace-sync.yaml	Updates OpenAPI specification with oldDoc parameter documentation and behavior table
db/crud.go	Renames SyncFnDryrun to SyncFnDryRun and fixes oldDoc body handling to use BodyBytes()