core: add AST-based linter for undeclared state reads in function-bas…

[Smitaambiger]

Closes #407

This PR adds AST-based validation for undeclared state reads in function-based actions.

What this does

• Parses the originating function using inspect.getsource
• Walks the AST to detect state["<literal>"] accesses
• Compares detected keys with declared reads=[...]
• Raises ValueError if undeclared keys are accessed
• Skips validation when source is unavailable

Why

Prevents silent bugs caused by accessing state keys that were not declared in reads, enforcing stricter action correctness.

Tests

• Added unit tests covering:
  • Valid declared reads
  • Undeclared read detection

[Smitaambiger]

@elijahbenizzy

Initial implementation for AST-based undeclared state read validation.

Currently implemented as a hard ValueError at action construction time.

Happy to adjust:

• validation timing (build-time vs decorator-time)
• severity (warning vs error)

Looking forward to feedback.