Fix incorrect JSON schema for FunctionCall and FunctionResponse

[felixarntz]

• FunctionCall and FunctionResponse are currently using oneOf, which is not correct. It is totally valid for a function call or a function response to have both id and name set. So this needs to be anyOf instead.
  • It doesn't show in any PHP-only usage, but as soon as you do something where the schema matters (e.g. REST API), this schema will lead to errors.
  • Regardless of whether or not it errors though, it is simply semantically incorrect.
• Somewhat related, the FunctionResponse was not handling its id and name the same way as FunctionCall. The properties weren't nullable, even though they should be - only one is required, though both are allowed. This was already reflected in parts of the class, but not consistently.

This PR fixes both of these bugs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.11%. Comparing base (211e603) to head (22d2d16).
⚠️ Report is 4 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #220      +/-   ##
============================================
+ Coverage     88.07%   88.11%   +0.03%     
- Complexity     1209     1212       +3     
============================================
  Files            60       60              
  Lines          3925     3928       +3     
============================================
+ Hits           3457     3461       +4     
+ Misses          468      467       -1     

Flag	Coverage Δ
unit	88.11% <100.00%> (+0.03%)	⬆️

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[github-actions]

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

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[gziolo]

Looks good to me! The oneOf → anyOf fix is semantically correct and the FunctionResponse alignment with FunctionCall resolves a real inconsistency. Well tested.

[gziolo]

Non-blocking nitpick (caught by agent): this fromArray() now uses array_key_exists with a defensive (string) cast, while FunctionCall::fromArray() still uses $array[self::KEY_ID] ?? null. The difference is subtle — array_key_exists distinguishes between a missing key and a key explicitly set to null. Neither causes a real-world issue, but it's a minor inconsistency between two sibling classes doing the same thing.

[felixarntz]

Great point, updated in 22d2d16.