perf(idkg): [CON-1575] reduce the number of signature share validations

[pierugo-dfinity]

This PR reduces the number of signature share validations from N to f+1, which are enough to aggregate the full signature. The strategy is very similar to the one done in #6526 to reduce the number of dealing supports: we introduce an in-memory map that keeps track for each RequestId, the set of share signers already validated. Once that set reaches a cardinality of f+1, we stop validating more signature shares for that RequestId. We follow the same approach as in #7069 to do all that in parallel: check with shared access to the lock, then perform the cryptographic validation and if valid, check again with exclusive access to the lock.

[Copilot]

Pull request overview

This PR optimizes the IDKG threshold signer path to reduce unnecessary signature share validations by tracking already-validated signers per request and skipping validation once the reconstruction threshold is met.

Changes:

• Renamed the IDkgMessage helper to sig_share_request_id_and_signer (and broadened its use for share bookkeeping).
• Added an in-memory cache (validated_sig_share_signers) to avoid re-validating duplicate shares and to drop excess shares after the reconstruction threshold is reached.
• Expanded test utilities and signer tests (including single-thread vs multi-thread behavior) and added a dev-dependency needed for the new tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
rs/types/types/src/consensus/idkg.rs	Renames/clarifies the signature-share helper used to extract (RequestId, NodeId) for bookkeeping.
rs/consensus/idkg/src/test_utils.rs	Adds thread-count configurable signer dependency constructors for deterministic testing.
rs/consensus/idkg/src/signer.rs	Introduces the validated-signer cache and uses it to reduce signature share validations; updates purge logic and adds/updates tests.
rs/consensus/idkg/Cargo.toml	Adds a dev-dependency required by the new signer tests.

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