docs: document callable/lambda guard conditions and add regression tests (#638)

[fgmacedo]

Closes #638.

Investigating the report, this is not a v3 regression: a lambda guard with a parameter named self fails identically on v2.6.0 and v3.2.0. Callbacks are invoked through dependency injection by parameter name, and self is only injected when the callable resolves to a bound method (i.e. it is an attribute of the state machine class). Plain callables (lambdas, module-level functions) are called unbound, so they must declare injectable names such as machine.

Since this behavior was neither tested nor documented, this PR:

• Adds TestIssue638 regression tests covering callable guards: lambda with machine, zero-arg lambda, lambda receiving event kwargs, lambda with self (asserts the current TypeError), and a class-body function passed by reference (bound, so self works).
• Adds a "Callable conditions" section to docs/guards.md with doctests and a warning explaining the self vs machine distinction.

Full analysis in #638 (comment).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (471fe17) to head (0da2b8f).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           52        52           
  Lines         5505      5505           
  Branches       869       869           
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  Hits          5505      5505           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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