Harden metrics disabled checks for caller/ancestor chains

[DylanBlakemore]

Description

• Prevent function_clause crashes by making ContextRegistry.disabled?/1 safely handle pids, atoms, nil, and unexpected terms.
• Normalize the caller/ancestor chain in Context.metrics_disabled?/0 by flattening and filtering to valid types before checking.
• Add regression tests covering nested $callers/$ancestors, mixed invalid terms, and atom ancestors.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my changes work
• New and existing unit tests pass locally with my changes