feat: implement all nine invalid and boundary test specifications for…

[D-Bluemoon]

… multisig operation

Closes #141

Description

Adds missing negative input and invalid-input validation coverage for MultiSigEscrowClient.initMultiSigOperation to prevent silent regressions across the seven parameter branches. Also implements critical boundary verification and unauthorized signer checks.

Tasks Status

• Covered missing negative test conditions inside _validateInitParams (including networkPassphrase mismatch and duplicate signers).
• Added valid boundary test case (threshold === signers.length).
• Added invalid boundary test case (threshold > signers.length).
• Implemented explicit validation test for unauthorized signers inside addSignature.

Closes #128

Description

Removes the dead legacy .eslintrc.json configuration file, which is completely ignored and unsupported by the current eslint@^10 toolchain. Authoritative rules have been successfully ported over to the active flat configuration file (eslint.config.mjs) to maintain maximum code quality standards without dropping baseline validation hooks.

Infrastructure Improvements

• Confirmed legacy configuration file was orphaned and unused by active linting pipelines.
• Ported critical rules (no-throw-literal, no-param-reassign, strict-boolean-expressions) directly into the modern flat-config array.
• Cleanly deleted the incompatible legacy .eslintrc.json file.
• Verified that only active modern flat-config infrastructure remains.

Closes #140

Description

Implements public export surface validation and automated documentation drift testing. This prevents future synchronization drift between the SDK entry point barrel surface (src/index.ts) and the public-facing documentation (docs/API.md), explicitly safeguarding critical exports like TrustFlowError.

Acceptance Criteria Status

• Added an explicit integration test in tests/integration.test.ts to validate a maintained allowlist of core exported items.
• Guaranteed strict barrel export visibility for TrustFlowError through explicit target assertions.
• Designed a lightweight, independent automation script (scripts/check-doc-coverage.js) to parse public module components and verify their presence within markdown documentation headers.
• Engineered automated coverage flags to block continuous integration pipelines with explicit exit code faults if documentation drift is detected.

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[D-Bluemoon]

Hi! I've implemented the public export surface integration test along with the automated document drift verification script. The changes have been pushed to this branch and are ready for workflow approval and review!