feat(engine): wire extensions and capabilities into runtime pipeline

[gouslu]

Change Summary

Part 4 of the Extension System (P1) series. Wires the previously
landed Capability Registry & Resolver (#2732) into the runtime
pipeline so extensions are actually instantiated, started, and
shut down by the engine, and so consumer nodes can resolve their
capability bindings at build time.

Highlights:

• Runtime wiring in runtime_pipeline.rs: extension lifecycle
  is invoked before any data-path node is spawned, and Shutdown
  is delivered to extensions only after the data path drains
  ("started first, shut down last"). Active and passive extensions
  are handled separately; failures abort startup cleanly.
• Local capability ownership aligned with shared via a
  Box-clone factory pattern, removing the prior asymmetry between
  the two trait variants.
• Two reference test capabilities under
  crates/engine/src/capability/: NoOpStateless and
  NoOpStateful. They exercise every codegen path of the
  #[capability] proc macro (&self × {sync, async}, &mut self
  × {sync, async}, borrowed/owned returns, etc.).
• Comprehensive end-to-end test suite at
  crates/engine/tests/extension_e2e.rs (26 tests) covering:
  passive/active/background extensions, lifecycle ordering and
  shutdown ordering, fail-fast on extension errors, dual-variant
  pruning, one-shot capability enforcement (all accessor
  combinations), shared mutable state across consumers via
  Arc/Rc for both local and shared trait variants, async
  &mut self invocation through boxed handles, and active
  extensions mutating shared state observed by capability
  consumers.
• Architecture doc updated with a precise statement of the
  start-first/shut-down-last invariant (it orders lifecycle
  calls, not init completion) and a noted future consideration
  to add an opt-in readiness probe if/when an extension needs an
  init-complete guarantee.
• URN unification: extension URNs now use the canonical
  4-segment form urn:<namespace>:extension:<id> (mirroring the
  receiver/processor/exporter convention), with a short form
  extension:<id>. The shared parser core lives in a new
  private crates/config/src/urn.rs; node_urn.rs and
  extension_urn.rs delegate to it with disjoint accepted-kind
  sets so the two URN types cannot be confused. As a consequence,
  NodeKind::Extension and the now-unreachable
  Error::ExtensionInNodesSection are removed. Misplacement
  errors include actionable hints (e.g. "declare under
  extensions: instead of nodes:").
• All in-tree node factories (receivers, processors, exporters
  in core-nodes and contrib-nodes) updated to accept the new
  &Capabilities parameter; existing factories that don't depend
  on any capability simply ignore it.

What issue does this PR close?

How are these changes tested?

• New extension_e2e.rs integration test (26 tests) exercises the
  wiring end-to-end against synthetic receivers/processors/
  exporters/extensions.
• New unit tests in urn.rs cover the shared parser core and the
  misplacement-error hints; existing extension_urn and
  node_urn tests updated to assert the canonical 4-segment form.
• Pipeline-level regression tests cover rejecting extension URNs
  in the nodes: section and node URNs in the extensions:
  section.
• cargo xtask check (structure check + fmt + clippy --workspace --all-targets -- -D warnings + cargo test --workspace) passes
  cleanly. No new clippy warnings.

Are there any user-facing changes?

Yes:

• Extension URN format: extension URNs now use
  urn:<namespace>:extension:<id> (4-segment) instead of the
  pre-existing 3-segment urn:<namespace>:<id>. Short form
  extension:<id> (expands to urn:otel:extension:<id>) is
  available as a developer convenience. Existing 3-segment
  extension URNs in pipeline configs must be updated; the bundled
  configs/fake-with-extension.yaml shows the new shape.
• New extension authoring surface: Extension trait,
  ExtensionWrapper::builder typestate, the
  extension_capabilities! macro, and the test capabilities
  NoOpStateless / NoOpStateful are now reachable for
  external extension authors. The architecture doc captures the
  lifecycle contract.
• Node factory signature now includes &Capabilities as a
  parameter; existing custom factories will need to accept (and
  may ignore) this new argument

[codecov]

Codecov Report

❌ Patch coverage is 86.33194% with 98 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.33%. Comparing base (0f3ffaf) to head (44ca58f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2860      +/-   ##
==========================================
+ Coverage   86.27%   86.33%   +0.06%     
==========================================
  Files         715      717       +2     
  Lines      272067   272391     +324     
==========================================
+ Hits       234715   235162     +447     
+ Misses      36828    36705     -123     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	87.31% <86.33%> (+0.07%)	⬆️
query_abstraction	80.61% <ø> (ø)
query_engine	90.73% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

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