Add fetch permission RPC method

[spencerstock]

What changed? Why?

Added a new RPC method coinbase_fetchPermission and corresponding public utility function to fetch a single spend permission by its hash. This enables retrieving specific permission details when you have a permission hash identifier.

Key changes:

• New RPC type definitions for coinbase_fetchPermission request and response
• New public utility method fetchPermission that accepts a permission hash and optional provider
• Integration in the Signer class to handle the new RPC method and store the fetched permission
• Exports added to both browser and Node.js entry points for the spend permission utilities

How was this tested?

The changes follow the existing patterns for spend permission methods in the codebase. All existing tests continue to pass:

• Linting passes with no new errors
• Type checking passes
• All unit tests pass (616 tests in account-sdk package)

How can reviewers manually test these changes?

1. Import the new fetchPermission method from the spend-permission utilities
2. Call it with a valid permission hash:
   • With a provider: await fetchPermission({ provider, permissionHash: '0x...' })
   • Without a provider: await fetchPermission({ permissionHash: '0x...' })
3. Verify the method returns the expected SpendPermission object with permission details
4. Confirm the permission is stored in the spend permissions store when it has a chainId

Demo/screenshots

[cb-heimdall]

✅ Heimdall Review Status

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Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[stephancill]

nice!