Fix yielded content rendered at wrong location with Rails form helpers

[joelhawksley]

Problem

When a ViewComponent renders a partial that uses a Rails form helper (like form.label) with a yielding block, the yielded content is rendered outside the helper's tag instead of inside it.

Expected: <label>world</label>
Actual: world <label></label>

Reported in #2617 — this PR provides the fix.

Root Cause

form_with creates form builders with @template_object pointing to the component. When the form builder is later used inside a partial (e.g. form.label :something do ... end), it calls @template_object.capture(builder, &block) — capturing on the component's @output_buffer.

However, Rails' Template#render → _run has already replaced view_context.@output_buffer with a fresh OutputBuffer for the partial. The compiled partial template code writes to this new buffer, while the component's @output_buffer still references the original one set during render_in.

This mismatch means form.label's capture reads from an empty (stale) buffer, and the block's output leaks into the partial's main output stream instead of being wrapped by the label tag.

Fix

Override capture in ViewComponent::Base to synchronize @output_buffer with view_context.output_buffer before delegating to super. This ensures form helpers that capture through the component use the same buffer that the block's template code writes to.

def capture(*, **, &block)
  old_output_buffer = @output_buffer
  @output_buffer = view_context.output_buffer if view_context
  super
ensure
  @output_buffer = old_output_buffer
end

Test

Adds a test case with a component that renders a partial using form.label with yield, verifying the yielded content appears inside the label tag.