Allow Dispatcher to inject a custom generator_class

[andriytyurnikov]

Summary

Adds an optional generator_class: keyword to Datastar::Dispatcher#initialize (defaulting to ServerSentEventGenerator). The three internal generator instantiations — stream_one, stream_many's connection generator, and each per-stream generator inside spawned threads — now go through the injected class.

Why

Downstream wrappers around Datastar (e.g. vibes-datastar) need to layer behavior on top of the SSE generator: input scrubbing, logging, metrics, debug capture, etc. Today the only option is monkey-patching ServerSentEventGenerator, because the dispatcher hard-codes the class at three call sites — and the user code inside Dispatcher#stream { |sse| ... } always receives an SDK-instantiated generator. Letting the dispatcher accept a subclass is the standard "let the wrapper override the workhorse class" hook.

This is the smaller, orthogonal change discussed in the comment thread on #18.

API

class ScrubbingGenerator < Datastar::ServerSentEventGenerator
  def patch_elements(elements, options = BLANK_OPTIONS)
    super(scrub(elements), scrub_options(options))
  end
  # ...
end

datastar = Datastar.new(
  request:, response:, view_context:,
  generator_class: ScrubbingGenerator,
)

Tests

4 new specs under `Dispatcher :generator_class`:

• defaults to `Datastar::ServerSentEventGenerator`
• one-off responses (`stream_one`) use the injected class
• multi-stream responses (`stream_many`) use it for the connection generator and every per-stream generator
• a subclass overriding `#write` observes every event written to the stream

Full suite: 104 examples, 0 failures.

Notes

• This is intentionally Dispatcher-local; no `Datastar.config` symmetry yet. Trivial to add later if needed.
• Related to SSE injection: bare \r and \n in scalar option values survive serialization #18. With this hook, downstream wrappers can ship their scrubbing fix today via a subclass while a canonical fix in `ServerSentEventGenerator` is pursued separately.

Test plan

• CI green
• Existing call sites still work (default behavior preserved)

[andriytyurnikov]

Upstream CI hasn't fired on this PR (workflow is on: push only). Fork CI run on the same commit, green: https://github.com/rubakas/datastar-ruby/actions/runs/24990202698 (Ruby 4.0 on Linux + full Datastar SDK conformance suite). 104 examples, 0 failures — the 4 new specs cover default class, stream_one, stream_many (connection generator + per-stream generators), and a subclass observing #write.