Keep Rails requests and their logs working with json 3 on Rails 8.0 and older

[PetrHeinz]

With json ≥ 3 on Rails 8.0 and older (Rails 7.2 before 7.2.4), every request of a Rails app using logtail-rails fails with a 500 and nothing is logged. ActiveSupport's encoder passes quirks_mode: to JSON.generate whenever to_json is called directly, and json 3 rejects unknown options. Evidence: Rails 7.1 and 8.0 apps with json 3.0.2 answered 500 to every request, and in the new spec here a request raises ArgumentError: unknown keyword: :quirks_mode from logtail-rack/http_request.rb:29.

logtail-rails itself has no to_json call in lib. All the calls on this path are in the gems it depends on:

• logtail-rack: headers_json of the request and response events (http_request.rb:29, http_response.rb:24), changed in Build headers_json with JSON.generate so json 3 can't fail every request logtail-ruby-rack#28;
• logtail: params_json (events/controller_call.rb:16), backtrace_json (events/error.rb:15) and every JSON log line (LogEntry#to_json, used by create_default_logger in the test environment and with LOGTAIL_SKIP_LOGS), changed in Encode JSON with JSON.generate so json 3 under ActiveSupport 8.0 can't break logging logtail-ruby#53.

So this PR adds the Rails-level regression test and, at merge time, the dependency floors:

• A spec dispatches a request and a raising request through the whole middleware stack, with ActiveSupport::JSON.encode stubbed to raise the way json 3 makes it raise. That covers every Rails version in CI, json 3 or not. It expects a 200, the request, controller call, render and response events with parseable headers_json and params_json, and an error event with backtrace_json, while the app's own exception gets through unchanged. Today the logger's ArgumentError replaces it.
• A third example logs a Time and NaN through Rails.logger with that encoder raising, and expects what ActiveSupport's to_json wrote before: "2016-09-01T12:00:00.000Z" and null. Plain JSON.generate would write Time#to_s and raise on NaN; logtail-ruby#53 converts with as_json first. There is no to_json or JSON.generate call in logtail-rails' own lib to convert the same way.
• On TruffleRuby, json's pure-Ruby generator writes Symbols and other non-JSON values with a stateless to_s.to_json, which goes back through ActiveSupport's encoder. The spec caught that on the TruffleRuby legs (the log level is a Symbol), and logtail-ruby#53 now converts with as_json before generating.
• The spec raises exceptions instead of rendering them for its own examples (show_exceptions = :none on Rails 7.1+), because since Rails 7.2 the spec app's false means :all.
• gemfiles/rails-7.1.gemfile and rails-8.0.gemfile keep their json < 3 pin. The spec app's own render json: hits the same ActiveSupport bug, so the stub is what covers json 3 there.

Behaviour and compatibility, once the fixed gems are in: headers_json, params_json and backtrace_json no longer escape HTML entities, so <, > and & (and U+2028/U+2029) stay as they are instead of \u003c, \u003e, \u0026. The JSON means the same. Everything else reads as before: logtail-ruby#53 converts values with as_json first, as ActiveSupport's to_json did, so times stay ISO 8601, NaN stays null and file uploads in params_json keep their hash form.

Targets the logtail-rails 0.2.15 patch release. It depends on logtail 0.1.21 (logtail-ruby#53) and logtail-rack 0.2.9 (logtail-ruby-rack#28):

• The TEMP: run CI against the logtail and logtail-rack branches until 0.1.21 and 0.2.9 are released commit points the root Gemfile and every gemfiles/*.gemfile at their claude/json-generate branches, so CI exercises the real fixes.
• At merge time, after both releases, replace that commit with logtail ~> 0.1, >= 0.1.21 and logtail-rack ~> 0.2, >= 0.2.9 in the gemspec, so that updating logtail-rails pulls in the fixed gems.

Commits and CI:

• Expect real response durations in the request specs is unrelated test maintenance that every logtail-rails PR needs right now: logtail-rack 0.2.8 times responses with the monotonic clock, which Timecop doesn't freeze, so three specs expecting "in 0.0ms" fail on every CI leg. It's identical to the same commit in Stop logging "Rendering" twice per template at debug level on Rails 7.2 to 8.1 #59, so the two merge cleanly in either order.
• The tests commit is expected to be red on every CI leg. The TEMP commit follows once both dependency branches carry their fixes, and the Time/NaN test is a follow-up commit on top.

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