perf(bridge): replace validator signature vecs with bitmap storage

[AJ0070]

Summary

This PR optimizes bridge validator signature tracking by replacing per-request Vec<AccountId> signer storage with a 256-bit bitmap representation.

Changes

• Added bitmap-backed signature storage for bridge requests using [u8; 32]
• Added stable validator bit-position tracking so historical request bitmaps remain readable
• Added get_signature_bitmap(request_id) for raw bitmap queries
• Added get_signer_list(request_id) for human-readable signer reconstruction
• Preserved backward compatibility by decoding legacy Vec<AccountId>-based requests
• Normalized legacy signature storage to bitmap form when legacy requests are mutated
• Updated bridge signing, execution, retry, and rollback flows to use bitmap helpers
• Moved bitmap coverage into contracts/bridge/src/tests.rs
• Fixed existing bridge test wiring so tests.rs is compiled and executed in the bridge crate

Backward Compatibility

Existing bridge requests stored with the legacy signature format remain readable.
New requests are stored using the bitmap representation.

Test Coverage

Added and/or validated tests for:

• bitmap signature tracking
• signer list reconstruction from bitmap
• signature count from bitmap matching monitoring/query results
• legacy signature format decode compatibility
• encoded-size benchmark comparison for 24 validator signatures
• existing bridge request and cross-chain status flows

Benchmark Note

The included test benchmark shows the bitmap encoding is smaller than the legacy Vec<AccountId> encoding for 24 validator signatures, which is the expected storage/gas improvement target for larger validator sets.

fixes #514

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[AJ0070]

@nanaf6203-bit Failures are not related to my changes.