Add terminal sync cancellation

[tszymczyszyn-shopify]

Summary

POS terminal cleanup needs a way to stop an in-flight v1 remote-replica sync without dropping the future and racing local replica deletion against libsql cleanup. This adds a cooperative cancellation path that returns through libsql's normal rollback and settlement flow.

Approach

• Track the active foreground sync with a per-sync cancellation token stored outside the replicator mutex.
• Thread cancellation through the replication state machine, remote waits, snapshot streaming, and SQLite injection.
• Interrupt and then join blocking SQLite injection work before returning cancellation.
• Preserve underlying injector errors that race with cancellation unless they are the expected SQLite interrupt/cancellation path.
• Re-check cancellation after successful injection/commit and before frame/snapshot loops return success, so terminal cancellation is not swallowed when SQLite finishes at the same time.
• Expose Database::cancel_current_sync_for_shutdown() for terminal shutdown callers.
• Treat shutdown cancellation as a retryable server replica event for the legacy libsql-server background replicator.
• Keep extension-test dependency resolution compatible with the Rust 1.85 CI toolchain.
• Add coverage for active public API cancellation, frame-fetch cancellation, snapshot request/pending/stream cancellation, injection rollback, and real SqliteInjector cancellation-await behavior.

CI follow-up

• Added tokio-util to the libsql replication feature so cargo udeps --manifest-path libsql/Cargo.toml --no-default-features --features replication can resolve the cancellation token dependency.
• Handled SyncCancelledForShutdown in the libsql-server replica loop to keep the new replicator error exhaustive in server builds.
• Pinned the standalone extension-test suite's transitive URL/idna dependency family to Rust 1.85-compatible releases after crates.io updates made fresh resolution select Rust 1.86+ crates.

Validation

• cargo fmt --all --check
• cargo check -p libsql_replication -p libsql --features replication
• cargo check -p libsql --no-default-features --features replication
• cargo check -p libsql-server
• cargo test -p libsql_replication cancellation_
• cargo test -p libsql_replication replicator
• cargo test -p libsql_replication sqlite_injector
• cargo test -p libsql_replication
• cargo test -p libsql --features replication cancel_current_sync_for_shutdown
• cargo test -p libsql --features replication
• (cd libsql-sqlite3/test/rust_suite && CMAKE_POLICY_VERSION_MINIMUM=3.5 cargo +1.85.0 test --features=extensions extensions --no-run)
• cargo clippy -p libsql_replication --tests --no-deps -- -D warnings -A clippy::needless-lifetimes -A clippy::suspicious-open-options -A clippy::nonminimal-bool

Strict cargo clippy -p libsql_replication --tests -- -D warnings and cargo clippy -p libsql --features replication --tests -- -D warnings remain blocked by pre-existing clippy warnings in libsql, libsql-ffi, and vendored code unrelated to this patch.

Co-authored-by: Claude noreply@anthropic.com
Orchestrated-by: ae noreply@shopify.com