### Does this PR have any customer impact?
### Type (Feature, Refactoring, Bugfix, DevOps, Testing, Perf, etc)
### Does it involve schema level changes? (Table, Column, Index, UDF, etc level changes)
### Are you introducing any new config? If yes, do you have tests with and without them being set?
### Release Note (Refer [Template](../docs/release_notes/pgmongo_template.md))
### Description
[schema validation] apply to stage $merge/$out
----
#### AI description (iteration 1)
#### PR Classification
New feature
#### PR Summary
This pull request introduces schema validation for the `$merge` and `$out` stages in MongoDB aggregation pipelines.
- Added `schema_validation_merge.md` to document the design and implementation details.
- Created new SQL functions in `bson_merge_functions--1.24-0.sql` to handle schema validation during the `$merge` stage.
- Updated `bson_aggregation_output_pipeline.c` to integrate schema validation logic into the `$merge` and `$out` stages.
- Modified `toc.yml` to include the new schema validation documentation.
- Updated `pgmongo--1.23-0--1.24-0.sql` and `pg_helio_api--1.23-0--1.24-0.sql` to include the new SQL functions.
### Does this PR have any customer impact?
No
### Type (Feature, Refactoring, Bugfix, DevOps, Testing, Perf, etc)
Refactor
### Does it involve schema level changes? (Table, Column, Index, UDF, etc level changes)
No
### Are you introducing any new config? If yes, do you have tests with and without them being set?
No
### Release Note (Refer [Template](../docs/release_notes/pgmongo_template.md))
### Description
Move source files under new directory.
----
#### AI description (iteration 1)
#### PR Classification
Code reorganization
#### PR Summary
This pull request moves the `helio_api` and `helio_distributed` sources under the `documentdb` directory, updating references and initialization logic accordingly.
- Added `pg_documentdb.c` for initializing the `pg_documentdb` extension.
- Added `documentdb_distributed.c` for initializing the `pg_documentdb_distributed` extension.
- Updated `validate_helio_extension_state.sh` to include new checks for `documentdb`.
- Moved `helio_api_extension_version.c` to `documentdb_extension_version.c` without code changes.
- Updated various `Makefile` configurations to reflect the new directory structure and dependencies.
Jia Hu
Vinod Sridharan