Add source-aware policy params declaration detection

[Maksym (maksym-mishchenko)]

Policy loaders need to identify policy-authored params rule heads before injecting assignment parameters; evaluating params misses conditional declarations. Add a nonexecuting HasPolicyParams(sourcePath) API across Rust, FFI, and C# that inspects only the uniquely matched loaded source. It detects default, function, set, and descendant rule heads without rejecting references or generated bridge rules, and reports missing, ambiguous, or unclassifiable sources as errors.

[Mark Birger (kusha)]

Maksym (@maksym-mishchenko) Could we make this check generic instead of hard-coding params? I’d call it HasDeclaredRuleRootedAt(sourcePath, rootName) and use HasDeclaredRuleRootedAt(sourcePath, "params") here. It should keep the current behavior: inspect declarations in that source, count params and its descendants, including defaults, without evaluating the rules.

[Anand Krishnamoorthi (anakrish)]

Maksym (@maksym-mishchenko) I second Mark's suggestion. It is better to make this generic. Also, ensure that we can detect rules that have prefixes. e.g metadata.parameters.

[Maksym (maksym-mishchenko)]

I changed it to HasDeclaredRuleRootedAt(sourcePath, rootName). "params" works as before. "metadata.parameters" matches that rule and its children, but not metadata.other. Defaults count too; the check doesn’t run the rules.