fix: implement buildSetOptionsOp helper for transaction builders

[Yusufolosun]

Closes #116

Summary

Adds a dedicated buildSetOptionsOp helper for transaction builders, covering signer updates, threshold updates, and master weight updates.

Root Cause

The transaction builder surface lacked a setOptions helper, leaving signer/threshold configuration with no reusable builder function and no focused unit coverage.

Fix

Added buildSetOptionsOp in builder.ts with:

• Signer key validation via existing validation utility
• Signer add/remove support (weight 0 removes)
• Threshold and masterWeight updates, including mixed input

Exported via index.ts.

Testing

Unit tests added in index.test.ts covering:

• Add signer

• Remove signer

• Threshold updates

• Mixed operations

• Invalid signer key validation

• ✅ lint passes

• ✅ type-check passes

• ✅ Unit coverage passes

• ✅ build passes

CI

Local checks match required CI stages defined in sdk-ci.yml: lint, type-check, unit tests with coverage, and build.

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[Yusufolosun]

Good day @amina69
I'm done with implementation and PR is up for review