Feature/unsupported errors

[Truphile]

I have completed the implementation and validation of the standard unsupported-feature and capability error
pattern across the SDK.

When I investigated the current branch, I found that the UnsupportedFeatureError and CapabilityMismatchError
error classes, capability registry, tests, and standard documentation were largely already present in the
codebase. However, there was a critical syntax bug in src/wallet/index.ts where a missing JSDoc closing tag (*/)
commented out necessary imports (including validatePublicKey). This was causing the testnet-funding capability
gating test to fail with a ReferenceError when falling back to the standard error response.

Here's a summary of the actions taken:

1. Checked out a new feature branch feature/unsupported-errors.
2. Verified the UnsupportedFeatureError type implementation (src/errors/unsupported.ts), which correctly
   incorporates module, operation, capability, and a registry-sourced developer hint.
3. Validated capability gating across experimental vault, soroban, wallet, and signer modules based on their
   status (e.g. config-gated, planned).
4. Fixed the missing JSDoc closing tag in src/wallet/index.ts that caused the module to crash before returning
   the CapabilityMismatchError.
5. Executed the unsupported-feature.test.ts unit tests to ensure that all standard behavior checks pass
   successfully.
6. Committed the fix to the branch. The updated codebase correctly provides detailed context on unsupported or
   missing features, and references the documentation (e.g., capability_error_standard.md) which explains supported
   vs. planned functionalities without overpromising dates.

closes #320