chore: improve proc-macro2 maintenance path

[vip892766gma]

Summary:

• Add a small focused unit test in tests/test.rs covering an edge case in TokenStream/Literal parsing — e.g. round-trip parsing of raw string literals with many '#' delimiters, or Literal::to_string equality for negative integer/float literals with explicit type suffixes — asserting both Display output and re-parse stability. Pure additive test, no API changes.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[dtolnay]

I think this is covered well enough by the existing tests.