fix(api): MultiBuilder rejects silently-dropped per-route consumer-wide settings

[eschizoid]

Summary

MultiBuilder.start() was a §12 silent-failure: per-route consumer-wide
settings like withRetry, withDeadLetterTopic, withErrorHandler,
withPollTimeout, withMetrics, withTracer, withCircuitBreaker,
and withBackpressure were quietly dropped when a route configurator
set them. The previous rejectPerRouteConsumerWideSettings guard only
caught withProcessingMode + withKeyOrderedMaxKeys; everything else
fell on the floor.

The bug

MultiBuilder.start() only invokes ds.buildPipeline() /
addBatchRoute(...) for each route. DefaultStream.applyCommonConsumerConfig
(which propagates retry / backpressure / metrics / errorHandler /
dlqTopic / pollTimeout / tracer onto the underlying KPipeConsumer.Builder)
is never called from the multi path. Every per-route consumer-wide
setting other than the two already guarded was silently lost.

Reproduction (before this PR)

KPipe.multi(props)
  .json("t", s -> s.withRetry(3, Duration.ofSeconds(1)).withDeadLetterTopic("dlq"))
  .start();

…built a consumer with neither retry NOR DLQ wired, and emitted no
error. The misconfig surfaced only as missing behavior at runtime —
exactly the overloaded-null / silent-drop footgun §12 exists to prevent.

Fix

Extend MultiBuilder.rejectPerRouteConsumerWideSettings to throw
IllegalArgumentException when a per-route configurator sets any
consumer-wide value. Two message shapes:

• Has a symmetric MultiBuilder.with* mirror (withMetrics,
  withTracer, withCircuitBreaker): error points users at the
  consumer-level setter.
• No mirror yet (withRetry, withBackpressure,
  withDeadLetterTopic, withErrorHandler, withPollTimeout):
  error points users at the explicit KPipeConsumer.Builder
  escape hatch and notes the deferred multi-builder mirror follow-up.

Deferred follow-up

Adding consumer-wide mirrors on MultiBuilder for withRetry /
withDeadLetterTopic / withErrorHandler / withPollTimeout /
withBackpressure is a separate change — they need design decisions
(e.g. should DLQ be a consumer-wide single topic or per-route routing?).
This PR's rejection message documents the gap so users hit a loud error
with a workable next step, instead of a silently-broken consumer.

Test plan

• One test per rejected setting: withRetry, withBackpressure,
  withDeadLetterTopic, withErrorHandler, withPollTimeout,
  withMetrics, withTracer, withCircuitBreaker. Each asserts the
  thrown IllegalArgumentException names the offending setter and the
  offending route topic, and (for mirrored settings) the message points
  at the consumer-level MultiBuilder.with* setter.
• Smoke test that the consumer-wide MultiBuilder setters
  (withMetrics, withTracer, withCircuitBreaker) still build
  cleanly together with the existing withProcessingMode /
  withKeyOrderedMaxKeys chain.
• ./gradlew :lib:kpipe-api:test — all 21 cases in
  KPipeFacadeBuildTest pass; full module suite green.

[eschizoid]

@copilot please review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.50%. Comparing base (4ccac98) to head (b1f8f06).
⚠️ Report is 9 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #199      +/-   ##
============================================
+ Coverage     77.82%   78.50%   +0.68%     
- Complexity      732      747      +15     
============================================
  Files            68       66       -2     
  Lines          2800     2815      +15     
  Branches        346      356      +10     
============================================
+ Hits           2179     2210      +31     
+ Misses          462      444      -18     
- Partials        159      161       +2     

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[eschizoid]

@copilot please review