[Rust] Harden OAuth token caching

[danilotrninic-db]

What changes are proposed in this pull request?

Parse the OAuth expires_in field from a quoted integer ("3600") in addition
to a plain JSON integer. Previously a token endpoint that returned expires_in
as a JSON string was read as "no lifetime reported," so the token was fetched
fresh on every stream creation instead of being cached.

How this addresses #607

#607 has three asks:

1. Parse quoted expires_in — the code change above.
2. Continue serving a valid cached token when a proactive refresh fails —
   already implemented in TokenCache::get_or_fetch; this PR verifies it and
   adds tests pinning it. The one edge case (a refresh that succeeds with no
   usable expires_in) is deliberately left returning the fresh token uncached
   rather than falling back to the near-expiry cached one — a missing lifetime is
   missing metadata about the token, not evidence it is bad, and the freshly
   minted token is the more likely of the two to still be valid.
3. Add deterministic tests — new TokenCache tests covering concurrency
   invalidation, cancellation, and expiry.

How is this tested?

cargo test --workspace (822 tests, green); cargo fmt --check and
cargo clippy clean. New unit tests cover parse_expires_in (integer, quoted
integer, whitespace, and reject cases) and the TokenCache behaviors above.

No live-server testing was needed: the sole production change is a pure parsing
function with no I/O or state, fully exercised by the unit tests above.