feat: Add automatic token refresh for API methods

[P24L]

• Add ensureValidToken() method to SessionConfiguration protocol
• Update prepareAuthorizationValue() to refresh tokens for auth-optional methods
• Modify getTimeline() as proof-of-concept for auth-required methods
• Tokens are refreshed if they expire within 10 seconds
• No breaking changes, maintains backward compatibility

Summary

This PR implements automatic token refresh functionality to prevent ExpiredToken errors when making API calls. The implementation follows the approach discussed in issue #203 and uses a helper method pattern to minimize code changes while maintaining backward compatibility.

Problem

• Access tokens expire after ~2 hours, causing API calls to fail with ExpiredToken errors
• Users currently must catch these errors and manually refresh tokens
• This creates poor developer experience and requires boilerplate error handling

Solution

Implemented automatic token refresh using a three-pronged approach:

1. Added ensureValidToken() helper method to SessionConfiguration

   • Checks if token expires within 10 seconds
   • Automatically calls refreshSession() if needed
   • Gracefully handles token parsing errors

2. Updated prepareAuthorizationValue() for auth-optional methods

   • Now calls ensureValidToken() before retrieving access token
   • All auth-optional methods automatically benefit from token refresh

3. Modified getTimeline() as proof-of-concept for auth-required methods

   • Added single line: try await sessionConfiguration?.ensureValidToken()
   • Demonstrates the pattern for other auth-required methods

Implementation Details

• Minimal code changes: Only 1 line per auth-required method
• No breaking changes: Fully backward compatible
• Centralized logic: Token refresh logic stays in SessionConfiguration

Next Steps

If this approach is approved, the same pattern can be applied to:

• ~167 other auth-required API methods

Linked Issues

#203

Type of Change

• Bug Fix
• New Feature
• Documentation

Checklist:

• My code follows the ATProtoKit API Design Guidelines as well as the Swift API Design Guidelines.
• I have performed a self-review of my own code and commented it, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings or errors in the compiler or runtime.
• My code is able to build and run on my machine.

[MasterJ93]

This is fine. I'll approve it. It'll be in the next update.