Add preview mode for update actions

[MrAnayDongre]

Summary

Adds preview mode for update actions in both the Python API and CLI.

With preview enabled, CocoIndex computes the same target actions as a normal update, but does not apply them to real targets and does not commit internal tracking state.

Closes #1890.

API and CLI

Preview is supported in:

• App.update(..., preview=True)
• App.update_blocking(..., preview=True)
• cocoindex update --preview

For the Python API, preview returns the planned action objects.
For the CLI, preview prints the planned actions.

Implementation

This reuses the existing planning path instead of introducing a separate planner.

In preview mode:

• reconciliation still runs through pre_commit(...)
• planning runs in a standalone write transaction
• that transaction is dropped without commit
• sink.apply(...) is skipped
• the later commit phase is skipped

This keeps preview aligned with normal update behavior while remaining read-only.

First pass scope

This first pass supports flat / leaf target actions only.

If a target requires child target providers, preview fails clearly instead of returning partial output.

CLI behavior

Added:

• cocoindex update ... --preview

Tests

Added coverage for:

• sync preview returns actions without writing
• async preview returns actions without writing
• preview rejects child target providers
• CLI preview prints planned actions

Validation

Ran locally:

• cargo fmt --check
• cargo test
• uv run mypy
• uv run pytest python/tests/core -q
• uv run pytest python/tests/internal -q
• uv run pytest python/tests/cli/test_cli.py -q