fix(keychain): show human-readable message for FailedPrecondition errors

[broody]

Summary

• Extract user-facing message from wrapped Coinbase API errors in the FailedPrecondition gRPC error handler
• Instead of showing "Service error: FailedPrecondition", displays the human-readable portion (e.g., "The user has reached the lifetime guest onramp transaction count limit (15)")
• Falls back to the full description if the pattern doesn't match

Test plan

• Trigger a FailedPrecondition error during Apple checkout (e.g., exceed Coinbase guest transaction limit) and verify the alert shows the readable message
• Verify other RPC error codes still display correctly

🤖 Generated with Claude Code

[vercel]

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Project	Deployment	Actions	Updated (UTC)
controller-example-next	Ready	Preview	Mar 18, 2026 9:52pm
keychain	Ready	Preview	Mar 18, 2026 9:52pm
keychain-storybook	Ready	Preview	Mar 18, 2026 9:52pm

[claude]

Claude finished @broody's task —— View job

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Documentation Analysis Complete

Tasks:

• Review the code change in packages/keychain/src/utils/errors.ts
• Assess if this requires documentation updates
• Search docs repo for relevant pages if needed
• Make updates if warranted
• Commit and push any changes

Change Summary:
The PR adds special handling for FailedPrecondition gRPC errors to extract human-readable messages from wrapped Coinbase API errors. Instead of showing "Service error: FailedPrecondition", users now see the actual error message (e.g., "The user has reached the lifetime guest onramp transaction count limit (15)").

Documentation Decision: No updates needed

This change is an internal implementation improvement that enhances the quality of error messages but does not warrant documentation updates because:

1. No API changes: The error handling behavior remains the same; only the message formatting improves
2. No new features: This refines existing functionality rather than introducing new capabilities
3. No user action required: Users don't need to configure anything or change how they use the SDK
4. Already documented: The existing Coinbase Onramp documentation already covers error handling conceptually at the appropriate level

The error message improvement makes the existing error handling better for end users, but it's an implementation detail rather than a user-facing feature change that requires documentation.

Following the directive to "DEFAULT TO NO CHANGES" - internal refactors and quality improvements do not require documentation updates unless they change APIs, configurations, or user-facing behavior in ways that users need to understand or act upon.

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