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Skip the query parse and the path concatenation in content negotiation (#2875)

`Formatter#negotiate_content_type` runs on every request and reads two things
before it can fall back to the API's declared format:

    fmt = format_from_extension || query_params['format'] || format || ...

`query_params` is `Rack::Request#GET`, so a request with no query string at all
still parsed one. And `format_from_extension` read `rack_request.path`, which is
`script_name + path_info` — a String built to be scanned once for a dot and
thrown away.

Both are now skipped in the common case. `?format=` cannot be present without a
query string, so an empty QUERY_STRING returns early. And the extension is the
tail of the path, so PATH_INFO answers it on its own whenever there is one: a
dot in SCRIPT_NAME is followed by the slash that opens PATH_INFO, and no
registered extension holds a slash. Only an empty PATH_INFO needs the
concatenation.

Measured in one process:

    rack_request.path        97 ns  ->  env[PATH_INFO]           39 ns   2.5x

and three fewer objects allocated per request on a bare JSON GET (46.1 -> 43.1),
counted deterministically end to end.

Both guards test `nil?`/`empty?` rather than `blank?` on purpose: neither value
has been scrubbed at this point, and `String#blank?` runs a regexp, which raises
on an invalid byte sequence — the case `formatter_spec.rb` covers with
`"/info.\x80"`.

Content negotiation is unchanged. Extension, `?format=`, declared format, Accept
header and default still resolve in that order, and a 21-case diff of responses
(extension, query, Accept casing, `*/*`, SCRIPT_NAME with a dot, empty
PATH_INFO, 204/404/405/406) is byte-identical to master.

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

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