### Does this PR have any customer impact? No
### Type (Feature, Refactoring, Bugfix, DevOps, Testing, Perf, etc) Feature
### 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
### ChangeLog (Refer [Template](../oss/CHANGELOG.md))
### Description
Support `let` variables and `$$NOW` in delete queries.
The values of the variables can be accessed in the query by using `$expr`.
Example:
```
{
delete: <collection>,
deletes: [
{
q: {$expr: {$eq: ["$_id", "$$varRef"]}},
limit: 0, // multi
}
],
let: {"varRef": 1},
...
}
```
We parse the `let` value (together with the time system variables - `$$NOW`) and propagate it through the aggregation and variable contexts.
----
#### AI description (iteration 1)
#### PR Classification
New feature
#### PR Summary
This pull request adds support for the `let` clause in delete operations.
- `delete.c`: Implemented logic to handle `let` clause in delete queries, including variable specification and collation.
- `bson_aggregation_pipeline_tests_let.out`: Added extensive test cases to validate the new `let` clause functionality in delete operations.
<!-- GitOpsUserAgent=GitOps.Apps.Server.pullrequestcopilot -->
Related work items: #3423118, #3613877
### Does this PR have any customer impact?
No
### Type (Feature, Refactoring, Bugfix, DevOps, Testing, Perf, etc)
Refactoring
### 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
Remove helio references from documentdb
Remove helio references from documentdb_distributed
----
#### AI description (iteration 1)
#### PR Classification
Code cleanup
#### PR Summary
This pull request removes references to "helio" and replaces them with "documentdb" in the sources of `documentdb` and `documentdb_distributed`.
- `metadata_cache.h` and `metadata_cache.c`: Replaced "Helio" with "DocumentDB" in variable names and function names.
- `pgmongo.c`: Updated include statements and function calls to use "documentdb" instead of "helio".
- `planner.c`: Updated planner function and related calls to use "DocumentDBApiPlanner".
- Moved several header files from `helio_*` to `documentdb_*` naming convention without code changes.
- `commands_common.c` and `create_indexes.c`: Updated function calls and variable names to use "DocumentDB" instead of "Helio".
Cornelius Boateng
Zinan Ren