[SizzleUnrlsd]

Closes #134 together with #138; this is the validator half.

Problem

healthchecks guards redirect(redirect_url) (hc/accounts/views.py:115) with _allow_redirect(redirect_url), a function of the same module that accepts relative URLs resolving to a known route. The refutation looks a Validator up by the callee's symbol, but a call to a function of the same module has none: its target is a known function, loaded as a global. A project therefore had no way to declare its own validator.

Change

• Project callees get a symbol. The refutation now names each call to a function of the analysed module by its project symbol (python.<module>.<qualified name>, as project_symbol builds it). A Validator, or an AuthorizationGuard, declared on that symbol applies in the function's own module, as it already did where the function is imported.
• Nothing changes by itself. The corpus snapshots are unchanged, because no bundled model names a project function. enterprise-validators (CoreTrace-Entreprise/coretrace-python-analyzer-plugins#25) will let a project declare them.
• Not done: validators still do not apply through summaries, to callers of a function that validates before its sink. No corpus case needs it: line 115 is a direct flow.

Verification

• pytest: 1068 passed. ruff check src tests and mypy are clean.
• pytest -m regression under Python 3.12: 19 passed, no snapshot changed.
• Tests. Before the change, a declared project validator left a hotspot, and a plugin declaring _allow_redirect on a healthchecks-like module left the open redirect. Both were red for that reason. An undeclared function, or one declared for another module, stays a hotspot.
• Mutation check. Going back to the call graph's symbols alone fails 2 tests.