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Look content types up in a plain Hash instead of an indifferent one (#2883)

`PrecomputedContentTypes#content_type_for` answers what media type a format
maps to, and backed that with a `HashWithIndifferentAccess`:

    def content_type_for(format)
      content_types_indifferent_access[format]
    end

Every read therefore ran `convert_key` before the lookup, because the callers
disagree on spelling: the format negotiated from a path extension, a `?format=`
or an `Accept` header arrives as a String, while the API's declared format and
the value stashed in `api.format` are Symbols.

Both spellings can be present in the same Hash instead. It is built once per
middleware and never written again, so the conversion is work that only ever
had one answer.

    HWIA[:json]     42.3 ns  ->  Hash[:json]     25.5 ns   1.66x
    HWIA['json']    38.8 ns  ->  Hash['json']    33.7 ns   1.15x

`content_type_for` is called twice on a request — once to negotiate the format,
once to set the response content type — and three times when the path carries a
format extension. That is about 34 ns per request, roughly 0.6% of a minimal
one. End to end it sits below what the version_throughput harness can resolve,
so no end-to-end figure is claimed here. Allocation counts are unchanged:
`convert_key` used `Symbol#name`, which allocates nothing.

Keys arrive as Symbols — the `content_type` DSL calls `key.to_sym` and the
defaults are Symbols — but the key is also stored as it came, so a middleware
constructed directly with String keys still answers to either spelling, as the
indifferent hash did. Checked against `HashWithIndifferentAccess` over the
default content types plus unknown, nil and empty keys, and over a string-keyed
hash looked up both ways: same answer every time.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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