feat(sdk-lib-mpc): add executeTillRound handler util

[vibhavgo]

Description

Adds an executeTillRound orchestration utility for the EdDSA MPSv2 DSG protocol, mirroring DklsUtils.executeTillRound for ECDSA DKLS. Also exposes getPartyIdx and getOtherPartyIdx accessors on the DSG class to support index-free wiring inside the utility.

• Add executeTillRound to modules/sdk-lib-mpc/src/tss/eddsa-mps/util.ts; accepts two un-initialized DSG instances plus key shares, message, and derivation path, calls initDsg internally using getPartyIdx(), and drives all 3 interactive rounds (WaitMsg1 -> WaitMsg2 -> WaitMsg3 -> Complete)
• Return intermediate DeserializedMessages[] for rounds 1-2 and the final 64-byte Ed25519 signature Buffer for round 3
• Add getPartyIdx() and getOtherPartyIdx() accessors to DSG class
• Add unit tests in modules/sdk-lib-mpc/test/unit/tss/eddsa/eddsa-utils.ts covering executeTillRound: per-pair signatures across all three 2-of-3 party combinations (0+1, 0+2, 1+2), root-key signature verification via getCommonKeychain(), derived-path (m/0/0) key isolation check, intermediate round return-type assertions, and out-of-range round error handling
• Migrate existing dsg.ts callers from the local runEdDsaDSG test helper to MPSUtil.executeTillRound and remove runEdDsaDSG from test/unit/tss/eddsa/util.ts
• Add dsg.ts protocol test asserting wasm-mps rejects cross-message signing with the wrapped "round WaitMsg2: Protocol Error" failure

Issue Number

WCI-386

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests added in modules/sdk-lib-mpc/test/unit/tss/eddsa/eddsa-utils.ts under the executeTillRound describe block, plus a cross-message protocol test in dsg.ts. Tests cover:

• Per-pair signature production for all three 2-of-3 party combinations (0+1, 0+2, 1+2), each verifying under the DKG public key
• Round 3 returns a 64-byte signature that verifies under the root public key from getCommonKeychain()
• Round 3 with derivation path m/0/0 produces a signature that does not verify under the root public key, confirming key derivation occurred
• Rounds 1 and 2 return DeserializedMessages[][] and not a Buffer
• Round values 0 and 4 throw Invalid round number
• Cross-message signing fails at round WaitMsg2 with the wrapped "Protocol Error" from wasm-mps
• Existing dsg.ts protocol, derivation-path, and session-management tests migrated to executeTillRound and continue to pass

Run with:
yarn run unit-test --scope @bitgo/sdk-lib-mpc -- -- --grep "executeTillRound"

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My code compiles correctly for both Node and Browser environments
• I have commented my code, particularly in hard-to-understand areas
• My commits follow Conventional Commits and I have properly described any BREAKING CHANGES
• The ticket or github issue was included in the commit message as a reference
• I have made corresponding changes to the documentation and on any new/updated functions and/or methods - jsdoc
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[linear-code]

WCI-386

[Marzooqa]

Implementation is correct, well-tested, and consistent with the ECDSA DKLS pattern. Two minor test-only nits as follow-ups — no public API impact.