[LOW] Handle incomplete Podcast Addict user agents

[OskarEichler]

Security impact (LOW)

A request-controlled User-Agent matching Podcast Addict can omit the expected comment block. Calling routine metadata, including to_h, then raises NoMethodError; repeated malformed requests can create avoidable application errors where parsed metadata is used.

This remains request-local and requires the application to parse and inspect the supplied header, so the impact is low.

Fix

Guard missing comments and OS data so optional Podcast Addict metadata remains nil instead of raising.

Verification

• Existing suite: 1,321 examples, 0 failures on Ruby 4.0.6 and 3.2.11.
• Focused malformed Podcast Addict, Dalvik, and Mozilla models return bounded metadata without exceptions.
• Combined targeted mutation pass: 2,966 variants derived from 175 existing corpus agents, 0 exception classes after all related robustness patches.

Breaking changes

None. Previously failing optional fields now return nil.