Add clustered collection tests

[danielfrankcom]

This change adds general tests for clustered collections.

The aim here is to test the runtime semantics unique to clustered collections like _id sort behavior, RecordId size limits, clustered-specific error codes for special numeric _id values, index immutability, and collection-level TTL management via collMod.

I renamed the directory from clustered-collections to clustered_collections to be a valid Python package, and consolidated everything under that directory. More extensive syntax tests for clustered collections with commands like create already live elsewhere in test files for those commands. The intent here is to only cover general semantic behavior of clustered collections that apply more generally.

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort L · Status Needs Review
Confidence: 0.90 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (504+0 LOC, 10 files); LLM: Adds new compatibility test cases covering clustered collection runtime semantics (sort, TTL, error codes, index immutability), scoped to the test-coverage component under documentdb_tests/compatibility/tests.

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