feat: expose writable subgraph_request_id and Context::remove for rhai-test (AS-389)

[andywgarcia]

Summary

Additive, test-enabling patches to the rhai-test fork line. Unblocks AS-389, requested by Indeed via TSH-22538.

Two Rust additions:

• Context::remove<K, V>(&self, key: K) -> Result<Option<V>, BoxError> — mirrors Context::get.
• subgraph::Response::set_subgraph_request_id(&mut self, id: SubgraphRequestId) — infallible setter; field remains pub(crate).

Two Rhai binding additions in plugins/rhai/engine/mod.rs:

• router_context::context_remove — surfaces as context.remove(key) in Rhai.
• set_subgraph_id_response — surfaces as response.subgraph_request_id = "..." on subgraph mocks.

Why

The mock subgraph Response exposed by apollo_mocks::get_subgraph_service_response() has a read-only subgraph_request_id, and Context has no public remove. This blocks unit testing of Rhai scripts that use the common correlation pattern (stash state keyed on subgraph_request_id in request.context, pull and remove from response.context).

Scope and non-scope

• No production routing behavior change.
• No changes to constructors, builders, field visibility, or any production call site.
• No setter for request.subgraph_request_id — Router owns that field on the request side.

Notes for reviewers

• Context::remove matches insert / upsert serde-failure semantics: if from_value::<V> fails on a present key, the original serde_json_bytes::Value is re-inserted under the same key before the error is returned, so callers that guess the wrong V don't silently lose data.
• That restore path has a brief non-atomicity window between the DashMap::remove and the re-insert. A concurrent writer who inserts a newer value under the same key during that window will have their write overwritten by our restore. Acceptable for this test-oriented fork; flagging for awareness if any of this ever heads upstream.

Test plan

• cargo build -p apollo-router
• cargo test -p apollo-router --lib context::test::test_context_remove
• cargo test -p apollo-router --lib context::test::test_context_remove_type_mismatch
• cargo test -p apollo-router --lib services::subgraph::tests::test_set_subgraph_request_id
• cargo test -p apollo-router --lib services::subgraph::tests::test_set_subgraph_request_id_empty_string_allowed
• Full lib suite passes: cargo test -p apollo-router --lib

Related

• AS-389
• TSH-22538
• Design: docs/superpowers/specs/2026-04-17-rhai-test-subgraph-request-id-context-remove-design.md (in productivity-dashboard repo)