CBG-5149: Stamp more default values into DefaultDbConfig() for visibility in include_runtime

[bbrks]

CBG-5149

Stamp several default values into DefaultDbConfig() for visibility in config requests specifying include_runtime

• Add test to catch future missed fields

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/267/

[Copilot]

Pull request overview

This PR addresses CBG-5149 by ensuring that default configuration values are properly stamped into DefaultDbConfig() so they are visible when users request configuration with the include_runtime parameter. The changes improve configuration transparency and add safeguards against future regressions.

Changes:

• Adds explicit default values to several previously unset fields in DefaultDbConfig()
• Introduces a comprehensive test that validates all DbConfig fields are either set to defaults or explicitly documented as intentionally unset
• Documents the reasoning for fields that intentionally remain unset through an allowlist with explanatory comments

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rest/config_database.go	Stamps default values for SessionCookieName, DisablePasswordAuth, BucketOpTimeoutMs, Guest, ChangesRequestPlus, and ImportPartitions (CE case)
rest/config_database_test.go	Adds TestDefaultDbConfigFieldCoverage to ensure all fields in DbConfig have either default values or documented reasons for being unset

[bbrks]

@RIT3shSapata I'm not 100% sure but those failing integration tests might be relevant to this change, so I'll take a first-look before your review.