feat: enable canister creation with subnet ID in the URL

[mraszyk]

This PR adds a new public HTTP endpoint /api/v4/subnet/<effective_subnet_id>/call on the replica that supports call to the create_canister endpoint of the management canister (aaaaa-aa). It is specified in this PR.

[Copilot]

Pull request overview

This PR adds support for a new replica HTTP update-call endpoint that is addressed by subnet ID (/api/v4/subnet/<effective_subnet_id>/call) and is restricted to aaaaa-aa (ic:00) create_canister calls, aligning with the referenced public spec change.

Changes:

• Introduces /api/v4/subnet/{effective_subnet_id}/call router/handler wiring in the public HTTP endpoints crate.
• Extends ingress validation to distinguish between canister-scoped vs subnet-scoped “effective destination” and enforces subnet endpoint restrictions.
• Adds/updates unit + system tests and a test-agent helper for exercising the new subnet endpoint; updates Bazel visibility/deps accordingly.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rs/http_endpoints/public/src/lib.rs	Wires a new v4 subnet call router into the main axum router.
rs/http_endpoints/public/src/call/call_sync.rs	Adds route + handler for /api/v4/subnet/.../call and refactors sync call handling to accept an “effective destination”.
rs/http_endpoints/public/src/call/call_async.rs	Updates async call path to use the new “effective destination” validation API.
rs/http_endpoints/public/src/call.rs	Adds EffectiveDestination and extends ingress validation to enforce subnet endpoint constraints.
rs/http_endpoints/test_agent/src/lib.rs	Adds CallSubnet test client for the new endpoint.
rs/http_endpoints/public/tests/common/mod.rs	Adds UpdateEndpoint to parameterize tests across canister vs subnet call endpoints.
rs/http_endpoints/public/tests/test.rs	Expands test matrix to cover subnet v4 sync calls + adds focused subnet endpoint tests.
rs/http_endpoints/public/tests/load_shed_test.rs	Extends load shedding tests to include the subnet v4 endpoint.
rs/tests/networking/http_endpoints_public_spec_test.rs	Adds system/spec test exercising subnet v4 behavior on a real topology.
rs/http_endpoints/test_agent/BUILD.bazel	Extends visibility to allow fuzz subpackages to depend on test_agent.
rs/http_endpoints/fuzz/BUILD.bazel	Adds dependency on the test_agent crate.

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