Test the ECDH participant gate that mutation testing found unguarded

[kwsantiago]

Summary

The check that a completed-secret announcement came from an actual participant in the ECDH session had no test. The function around it reads as covered: there is a test named for rejecting an unknown session, and it does reject one, but it returns at the session lookup twenty lines earlier and never reaches the authorisation that follows. Coverage of the function was not coverage of the gate inside it.

Mutation testing is what surfaced it rather than reading. Two mutations of that gate survived a previous run: inverting the pubkey comparison, so it matches every peer except the real one, and deleting the negation on the result, so participants are refused and everyone else admitted. Either one hands a derived shared secret to a key that is not in the session, and nothing in the suite objected to either.

No behavior changes here. This is the test that was missing.

Test plan

Two tests, and the second is what makes the first mean anything. With no peer registered the lookup returns nothing and every sender fails the gate, so a refusal on its own would prove only that something refused, not that identity was checked. The first sends a key that is not a participant and requires refusal; the second sends the registered participant's key with a deliberately wrong-length secret and requires the failure to move past the gate to the length conversion, which shows the gate discriminates on identity rather than rejecting whatever reaches it.

Verified against the mutants rather than assumed: both survivors are killed by these tests, checked by applying each mutation directly and confirming the suite goes red, then restoring. The whole crate's library tests pass, formatter and clippy clean.

Worth noting for anyone extending this: the crate's integration tests need the testing feature, so a bare cargo test -p keep-frost-net fails on unrelated unresolved imports before reaching anything here.

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Reviewing files that changed from the base of the PR and between 11a9a2a and dba2acf.

📒 Files selected for processing (1)

• keep-frost-net/src/node/ecdh.rs

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