fix: throw SdkError for unsupported capabilities in assertCapability()

[kai-agent-free]

Summary

Fixes #430

The assertCapability() method in Client was throwing a generic Error when a server doesn't support a required capability. This made it impossible for callers to programmatically distinguish unsupported-capability errors from other failures without fragile message-string matching.

Changes

• packages/client/src/client/client.ts: Replace throw new Error(...) with throw new SdkError(SdkErrorCode.CapabilityNotSupported, ...) in assertCapability(). This is consistent with assertCapabilityForMethod() which already uses SdkError with the same error code.

• test/integration/test/client/client.test.ts: Add assertions verifying that the thrown error is an instance of SdkError with code: SdkErrorCode.CapabilityNotSupported.

Usage

Callers can now catch unsupported capability errors programmatically:

try {
  await client.listPrompts();
} catch (error) {
  if (error instanceof SdkError && error.code === SdkErrorCode.CapabilityNotSupported) {
    // Handle unsupported capability gracefully
  }
}

All 386 tests pass.

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Latest commit: 95e2a96

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