fix: implement validation and fund transfer for release_partial

[lishmanTech]

Summary

This PR fixes release_partial, which was previously a public no-op that returned Ok(()) without releasing funds, validating the escrow, checking authorization, updating state, or emitting a release event.

The implementation now makes release_partial behave consistently with the existing release flows.

What changed

• Implemented buyer authorization for release_partial

• Added escrow existence validation

• Added amount validation for:

  • amount <= 0
  • amount > remaining escrow balance

• Reused the existing fee calculation helper

• Transferred the requested partial amount

• Persisted the reduced escrow balance

• Transitions the escrow to Released when the remaining balance reaches zero

• Emits FundsReleasedEvent

• Updates total released amount and release count

• Preserved the existing partial_releases_enabled feature-flag check

• Added documentation for release_partial, including its arguments and errors

• Defined and tested interaction between release_partial and release_item to prevent over-release

• Corrected the stale README claim regarding refund_escrow

• Audited the remaining public entry points for unused parameters/silent no-op implementations

Tests

Added coverage for the enabled release_partial path, including:

• Non-existent escrow → EscrowNotFound
• Zero/negative amount → InvalidEscrowAmount
• Amount greater than remaining balance → InvalidEscrowAmount
• Non-buyer caller → Unauthorized
• Correct fee handling
• Correct balance/state updates
• Release event and counters
• Interaction with release_item
• Existing disabled feature-flag behavior remains unchanged

Issue

Closes #271

CI

All required CI checks must pass:

• Formatting
• Clippy
• Build Optimized WASM
• Tests
• Dependency Audit
• Coverage
• SDK Error Code Parity