refactor(runtime): extract PushRouter transport seam behind StreamingDispatch trait

[tanmayv25]

Summary

Extract the final-hop transport in PushRouter behind a new
StreamingDispatch<T, U> trait. After PushRouter selects a worker, the
final hop goes through this trait object instead of a fixed
AddressedPushRouter. PushRouter keeps instance selection, occupancy, fault
detection, and migration; only the transport below the seam can now be swapped.
AddressedPushRouter (the request plane) is the default implementation and its
behavior is unchanged.

This is a behavior-preserving refactor with no new functionality — it makes
the final-hop transport pluggable behind a single typed seam. No alternate
transport implementation is included here.

Changes

• New StreamingDispatch<T, U> trait (generate, generate_bidirectional,
  on_instance_removed / on_instance_added), with AddressedPushRouter as the
  default impl delegating to its existing AsyncEngine path.
• PushRouter.addressed: Arc<AddressedPushRouter> -> Arc<dyn StreamingDispatch<T, U>>.
• spawn_instance_removal_watcher is generic and drives discovery-removal
  cleanup through the trait.
• New PushRouter::from_client_with_dispatch to inject a custom dispatch
  (unused in this PR).
• Rename inherent AddressedPushRouter::generate_bidirectional ->
  dispatch_bidirectional (disambiguates from the trait method); make
  AddressedRequest::into_parts public.

Validation

• cargo build -p dynamo-runtime and -p dynamo-llm (downstream) compile.
• cargo test -p dynamo-runtime egress suite: 67 passed, 0 failed — the
  request-plane path is exercised unchanged.

---

Enables pluggable final-hop transports behind the router seam for future work.

[datadog-official]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 43.93% (-5.92%)

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[devin-ai-integration]

Devin Review found 2 potential issues.

[coderabbitai]

Walkthrough

Changes

Streaming dispatch integration

Layer / File(s)	Summary
Streaming dispatch contract lib/runtime/src/pipeline.rs, lib/runtime/src/pipeline/network/egress/addressed_router.rs	Exports StreamingDispatch, makes AddressedRequest::into_parts public, renames bidirectional dispatch, and defines the dispatch lifecycle interface.
Addressed router implementation lib/runtime/src/pipeline/network/egress/addressed_router.rs	Implements unary and bidirectional dispatch plus instance removal and addition handling for AddressedPushRouter.
PushRouter dispatch wiring lib/runtime/src/pipeline/network/egress/push_router.rs	Uses type-erased dispatch implementations, routes discovery callbacks through them, and adds construction with a caller-supplied dispatcher.

Estimated code review effort: 3 (Moderate) | ~25 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the refactor and validation, but it omits the required Related Issues section and reviewer starting point.	Add the required Related Issues section with either an issue reference or a confirmed no-issue checkbox, and identify where reviewers should start.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the extraction of the PushRouter transport behind the StreamingDispatch trait.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Fix failing CI checks

---

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/runtime/src/pipeline/network/egress/push_router.rs`:
- Around line 326-333: The watcher deduplication in
spawn_instance_removal_watcher must account for dispatch identity, not only
EndpointId. Enforce a single StreamingDispatch per endpoint or update
ENDPOINT_WATCHER_ACTIVE and its ownership checks to distinguish dispatch
instances, ensuring every independent dispatch receives on_instance_removed and
on_instance_added callbacks.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 8fb16d73-7b31-48de-a591-56e887a90d69

📥 Commits

Reviewing files that changed from the base of the PR and between 6cc7c4e and d21cfc7.

📒 Files selected for processing (3)

• lib/runtime/src/pipeline.rs
• lib/runtime/src/pipeline/network/egress/addressed_router.rs
• lib/runtime/src/pipeline/network/egress/push_router.rs

[PeaBrane]

✅ AI-assisted review complete. Approving with two non-blocking comments for human evaluation.