diagnostics_channel: add opt-in subscriber suppression via suppressedBy and suppressed()

[DivyanshuX9]

Summary

This change moves suppression tracking into the diagnostics channel runtime
using a per-async-context storage so suppression state survives Promise/timer
boundaries. It introduces a lazy AsyncLocalStorage-based context for suppression,
wires per-subscription/store opt-in, and exposes a suppressed(key, fn, thisArg, ...args) helper that runs fn with the given suppression key active for the
current async context.

---

What Changed

lib/diagnostics_channel.js

• Use lazy-initialized AsyncLocalStorage for suppression context
• Implemented withSuppressionsContext(set, fn, thisArg, args)
  and getSuppressionsStorage()
• suppressed(key, fn, ...) now runs fn inside the ALS context so
  suppression persists across Promise/async boundaries
• ActiveChannel.prototype.subscribe and bindStore accept an
  options.suppressedBy opt-in and validate key types
• publish and store-scoping now check the active suppression set
  (via ALS.getStore()) and skip subscribers/stores whose
  suppressedBy key is active

Tests

• Added test/parallel/test-diagnostics-channel-suppression.js
  covering 10 scenarios: sync, async, timer, and store cases

Note: A temporary debug console.error in suppressed() was added
during development and must be removed before merge.

---

Why

The previous stack-based approach lost suppression state across async
boundaries (Promise and timer continuations). Using AsyncLocalStorage
preserves suppression state across Promise chains and
microtask/macrotask boundaries while remaining safe to lazily initialize
during bootstrap.

---

Backward Compatibility

• Suppression is opt-in per-subscriber and per-store — existing code
  is completely unaffected unless it explicitly uses suppressedBy or
  calls suppressed()
• ALS initialization is lazy and wrapped in try/catch — if ALS is
  unavailable at runtime, the code falls back gracefully to a non-ALS
  path with no cross-async persistence, preventing snapshot/bootstrap
  failures
• Behavior changes are strictly limited to code that uses
  suppressedBy or calls suppressed()

---

Testing Performed

• Rebuilt Node so snapshot includes the modified JS
• Ran python tools/test.py parallel/test-diagnostics-channel-suppression.js
• Verified suppression across Promise and timer boundaries via standalone scripts
• One harness mustCall mismatch was observed during debugging (now resolved)

Recommend a full CI run before landing.

---

Pre-Merge Checklist

• Remove temporary console.error debug logging from suppressed()
  in diagnostics_channel.js
• Run full test suite — at minimum:
  tools/test.py parallel/test-diagnostics-channel-suppression.js —
  until all tests pass with no harness warnings
• Clean up test-diagnostics-channel-suppression.js
  (remove any debug scaffolding used during development)
• Update API docs — document:
  • suppressed(key, fn, thisArg, ...args)
  • subscribe(handler, { suppressedBy })
  • bindStore(als, transform, { suppressedBy })
• Add changelog entry in CHANGELOG.md per repo process
• Run microbenchmarks on heavily-subscribed channels to
  verify ALS overhead is acceptable
• Request reviews from: @nodejs/diagnostics @nodejs/async-hooks
• Squash/clean commits before landing if preferred

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[DivyanshuX9]

@BridgeAR @bengl @Qard , I am opening this as a implementation
of #63623. Test file is included. Happy to adjust the API shape
or implementation based on your feedback before CI runs.

[codecov]

Codecov Report

❌ Patch coverage is 97.11538% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.31%. Comparing base (dbec31c) to head (e4aea85).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_errors.cc	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63651      +/-   ##
==========================================
+ Coverage   90.28%   90.31%   +0.03%     
==========================================
  Files         730      732       +2     
  Lines      234802   236480    +1678     
  Branches    43953    44527     +574     
==========================================
+ Hits       211991   213580    +1589     
- Misses      14530    14612      +82     
- Partials     8281     8288       +7     

Files with missing lines	Coverage Δ
lib/diagnostics_channel.js	98.34% <100.00%> (+0.20%)	⬆️
src/node_errors.cc	63.17% <0.00%> (+0.10%)	⬆️

... and 36 files with indirect coverage changes

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