Squash assorted Clippy and Rust doc warnings in codegen integration tests (#3684)

This commit gets rid of some Clippy and Rust doc warnings we produce in
generated code, which are surfaced in our codegen integration tests. The
aim is that eventually we will be able to deny Clippy and Rust doc
warnings in #3678 (despite us baking in `RUSTDOCFLAGS="-D warnings"` and
`RUSTFLAGS="-D warnings"` in our CI Docker image, we curently clear
these in codegen integration tests). Note that denying compiler warnings
is separately tracked in #3194.

The commit contains fixes for the following:

- Unconditionally referring to the `crate::types` module in Rust docs
when
  the Smithy model is such that it is not generated.
- Broken intra-crate doc links for client documentation.
- Incorrectly escaping Rust reserved keywords when referring to
  operation names in Rust docs.
- An unnecessary semi-colon when rendering additional client
  plugins.
- Not escaping brackets in text when rendering Rust docs containing
  HTML, that are interpreted as broken links.
- A broken link to unit variants of unions.
- Using `TryFrom` instead of `From` when infallibly converting from
  `&str` to `String` when deserializing an `@httpPayload` in the server.
- Using a redundant closure with `unwrap_or_else` when falling back to a
  `@default` boolean or number value for a `document` shape, instead of
  `unwrap_or`.
- Not opting into `clippy::enum_variant_names` when rendering the
  `Operation` enum in the server (since we render the operation names
  that were modeled as-is).

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