Summary

Two related defensive fixes to source-adapter / SPI infrastructure.

ProcessAdapter::call is not safe for concurrent use. It acquires the stdin Mutex to write the request, releases it, then acquires stdout to read the response. Two concurrent callers can therefore interleave on the JSON-RPC pipe; the per-call id-mismatch check turns the race into a hard error rather than silent corruption, but the entire process-adapter SPI is effectively single-threaded per instance — undocumented. Adds a call_lock: Mutex<()> held across the entire write→read pair so calls serialize cleanly, and documents the constraint on the function. The id-mismatch check stays as a defence against a misbehaving adapter.

Source-adapter HTTP clients leaked user-supplied IDs into URL paths via format!() without percent-encoding. Affects:

• rocky-fivetran — group_id and connector_id in /v1/groups/{}/connectors, /v1/connectors/{}, /v1/connectors/{}/schemas (interpolated in connector.rs / schema.rs; the client.rs GET helper just stitches the path onto base_url).
• rocky-airbyte — id in GET /v1/connections/{}.
• rocky-iceberg — namespace parts in GET /v1/namespaces/{}/tables. The existing replace('.', "%1F") is the spec separator, not encoding; the parts themselves were unencoded.

Fix: a small percent-encoding helper in each crate using RFC 3986 unreserved chars minus . (so .. cannot traverse, while ordinary identifiers like my_id-v2 round-trip cleanly). For Iceberg the order is split → encode each part → join with %1F; encoding after the join would re-encode the separator's %.

Adds percent-encoding to [workspace.dependencies] and to each of the three source-adapter crates.

Test plan

• cd engine && cargo test -p rocky-adapter-sdk -p rocky-fivetran -p rocky-airbyte -p rocky-iceberg --features rocky-fivetran/test-support — green
• cd engine && cargo clippy -p rocky-adapter-sdk -p rocky-fivetran -p rocky-airbyte -p rocky-iceberg --all-targets --features rocky-fivetran/test-support -- -D warnings — clean
• cd engine && cargo fmt --check — clean
• cd engine && cargo build --workspace — clean

New tests:

• process::tests::test_concurrent_calls_do_not_swap_ids (cfg(unix)) spawns a Python-based JSON-RPC echo adapter and tokio::join!s two calls with distinct tags; asserts each caller receives its own tag/id back. Skips silently if python3 is unavailable.
• wiremock_tests::test_list_connectors_percent_encodes_group_id, test_get_connector_percent_encodes_connector_id, test_get_schema_config_percent_encodes_connector_id — wire-level Fivetran tests where the mock matches only on the encoded path.
• Airbyte unit test asserting get_connection's URL escapes /, ?, #, .., and %.
• Iceberg unit tests covering each metacharacter per part, the ..-via-split case, and the multi-level "encode-then-join" ordering invariant.