Validate dry_run update mode

[sueun-dev]

This fixes dry_run=True being sent with ID-based vector updates.

What changed:

• Reject dry_run=True when id is used in sync REST, async REST, and gRPC update paths.
• Clarify the update docstrings so dry_run is described as filter-only.
• Add unit coverage for sync, async, and gRPC validation.

Why:
The update proto documents dry_run as returning the count of records that match the filter. ID-based updates do not have supported dry-run semantics, so the SDK should fail before sending that request.

Checked:

• On current origin/main, idx.update(id="vec1", set_metadata={...}, dry_run=True) sends {"id": "vec1", ..., "dryRun": true} instead of raising.
• On this branch, the same call raises ValidationError before any API call.
• PROTOC=$PWD/.venv/bin/protoc .venv/bin/python -m pytest tests/unit/test_index_update.py tests/unit/test_async_index_operations.py tests/unit/test_grpc_index.py -q (143 passed)
• .venv/bin/ruff check . (All checks passed!)
• PROTOC=$PWD/.venv/bin/protoc .venv/bin/python -m pytest tests/unit -q -k "not test_async_transport_calls_module_level_compute_backoff and not test_linux_keepalive_params and not test_darwin_keepalive_params" (4601 passed, 30 skipped, 3 deselected, 1 xfailed)

Note: a full local tests/unit run on macOS/Python 3.12 still has unrelated failures in test_socket_options for missing socket.TCP_KEEPIDLE; the same socket checks fail on current origin/main in this environment.

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Note

Low Risk
Small, consistent input validation across three update entry points; filter-based dry-run is unchanged and tests were added.

Overview
update() now rejects dry_run=True when an id is used, across sync REST (Index), async REST (AsyncIndex), and gRPC (GrpcIndex). The SDK raises ValidationError with "dry_run is only supported for filter-based updates" before any request is sent, instead of posting payloads like {"id": "...", "dryRun": true}.

Docstrings for dry_run now state it applies only to filter-based updates. Unit tests cover the new validation on sync, async, and gRPC paths; filter-based dry-run behavior is unchanged.

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