feat: Support collection group

[maruware]

No description provided.

[Copilot]

Pull request overview

Adds support for Firestore collection group operations across parsing, execution, tab-completion, tests, and documentation, enabling queries/counts that target all collections with a given collection ID across the DB hierarchy.

Changes:

• Introduces a COLLECTION_GROUP keyword/token and parsing support for QUERY and COUNT.
• Extends QueryOperation / CountOperation to track collection-group mode and updates the executor to run collection-group queries.
• Updates docs and adds parser/executor/completer test cases for collection-group usage.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
token.go	Adds COLLECTION_GROUP token/keyword mapping.
parser.go	Parses QUERY/COLLECTION_GROUP <id> and COUNT/COLLECTION_GROUP <id>; validates group name.
parser_test.go	Adds parse tests for collection-group query/count.
operation.go	Adds collectionGroup flag + constructors/accessors for query/count operations.
executor.go	Executes collection-group query/count paths in Firestore.
executor_test.go	Adds executor tests for collection-group query/count behavior.
docs/operations.md	Documents collection-group syntax, examples, and constraints.
completer.go	Adds COLLECTION_GROUP completion support for query/count.
completer_test.go	Adds completion tests around COLLECTION_GROUP keyword flows.

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