feat(signing): validate key format before signing in KeypairSigner (#768)

[therealbibson]

Overview

Validates Stellar secret key and Keypair format at KeypairSigner construction time to surface format errors eagerly, rejecting malformed secret keys with InvalidKeypairError.

Related Issue

Closes #768

Changes

Signing Key Vault Adapter

• [MODIFY] src/signing/adapters/KeypairSigner.ts
  • Added upfront secret key validation in constructor using Keypair.fromSecret
  • Validated that Keypair instances have signing capability (canSign())
  • Throws InvalidKeypairError when secret key or keypair format is invalid
• [MODIFY] src/errors.ts
  • Defined InvalidKeypairError and isInvalidKeypairError type guard
• [MODIFY] src/index.ts
  • Exported InvalidKeypairError and isInvalidKeypairError
• [MODIFY] test/signing.test.ts
  • Added unit tests for secret key validation, format errors, public-key-only keypairs, invalid types, and error type guards
• [MODIFY] test/sdkExports.test.ts
  • Added assertions verifying public API export of InvalidKeypairError and isInvalidKeypairError

Verification Results

 RUN  v1.6.1 /Users/user/Documents/Projects/open_source/split-sdk

 ✓ test/signing.test.ts  (20 tests) 135ms
 ✓ test/sdkExports.test.ts  (2 tests) 2ms

 Test Files  2 passed (2)
      Tests  22 passed (22)

Acceptance Criteria	Status
The constructor validates that the provided secret key is a valid Stellar secret key (starts with 'S', correct length, valid base32)	✅ Validated via Keypair.fromSecret at construction time
An InvalidKeypairError is thrown at construction with a message indicating the format issue	✅ InvalidKeypairError thrown with descriptive format error message
Valid keys continue to work unchanged	✅ Valid secret key strings and Keypair instances construct and sign as expected
Unit tests pass	✅ All 20 signing tests and 2 sdkExports tests pass

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