dev: Use expect rather than allow

[LukeMathWalker]

To prevent stale #[allow(...)]s, this PR configures clippy to reject them in favour of the #[expect(..., reason="...")] attribute.

In the process, it performs a few clean-ups:

• All allows for panic/expect/unwrap in tests are removed. They are unnecessary, given our clippy.toml file:

  allow-unwrap-in-tests = true
  allow-expect-in-tests = true
  allow-panic-in-tests = true

• (Plausible) reasons are added where they were missing
• All lints are set to warn. No need to use deny, since we fail on warnings in CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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