[dajiaohuang]

Closes #1708

Rationale

enable_url_table() previously returned a handle backed by a separate SessionContext allocation while retaining the session ID. This allowed live handles to diverge in state and could invalidate weak FFI task-context providers when callers reassigned the returned handle.

Changes

• Rebuild the URL-table catalog through the existing session's state_ref() and return a handle sharing the same Arc<SessionContext>.
• Preserve the session ID and make repeated calls idempotent.
• Add Python state-sharing and FFI-provider regression coverage, and update the user guide, extension/contributor documentation, and FFI capsule protocol guidance.

User-facing change

Both the original handle and the returned handle now observe URL-table enablement and subsequent state changes. Reassigning the result remains valid. This is a behavioral API change; the api change label is required. I attempted to apply the label, but GitHub denied AddLabelsToLabelable for this account, so the label remains unapplied.

Validation

• cargo check --features substrait --package datafusion-python — passed.
• Focused Python and FFI regression tests — 2 passed.
• pytest python/ on Windows — 1,626 passed, 11 skipped, 2 failed. The failures are in the read_arrow and register_arrow doctests: TemporaryDirectory cleanup raises WinError 32 while data.arrow is still held open.
• Selected pre-commit hooks passed: Ruff, Ruff format, ty, Rust fmt, and codespell.