Add Async.queueable() no-arg builder for conditional chaining

[Mateusz7410]

Summary

• Add Async.queueable() no-arg overload + matching QueueableBuilder() constructor so jobs can be chained incrementally
• chain(QueueableJob) skips the implicit chain() step when the builder has no current job, so the first .chain(job) on an empty builder becomes the first job in the chain
• enqueue() on an empty builder returns a Result with null salesforceJobId and an empty QueueableChainState (no-op, no exception)
• Document the new entry point + add Result.job and clarify the salesforceJobId description in website/api/queueable.md
• Add 4 new AsyncTest methods: empty enqueue, single chain on empty builder, multi-chain via repeated .chain(), and conditional-branch chaining

Why

Closes #28. Lets callers build a chain whose membership depends on runtime conditions without workarounds:

QueueableBuilder builder = Async.queueable();
if (cond1) { builder.chain(new Job1()); }
if (cond2) { builder.chain(new Job2()); }
if (cond3) { builder.chain(new Job3()); }
builder.enqueue(); // enqueues whatever was added; no-op if nothing was

Test plan

• bash scripts/agent/verify.sh full (CI-parity local Apex tests + coverage)
• bash scripts/agent/verify.sh ns (btcdev-namespaced scratch — package-install perspective)

Files

• force-app/main/default/classes/Async.cls (+4)
• force-app/main/default/classes/queue/QueueableBuilder.cls (+11, -2)
• force-app/main/default/classes/AsyncTest.cls (+90, -1)
• website/api/queueable.md (+73, -35)

Closes #28

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Project	Deployment	Actions	Updated (UTC)
async-lib	Ready	Preview, Comment	May 17, 2026 6:52pm

[github-actions]

🧪 Apex Test Results

✅ All Tests Passed

==========================================
     APEX TEST EXECUTION SUMMARY
==========================================

📊 Total Tests: 89
✅ Passed: 89
❌ Failed: 0
⏭️  Skipped: 0


🎉 All tests passed successfully!

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---

📊 Stats: 89 total | ✅ 89 passed | ❌ 0 failed
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (2ff435d) to head (9908a89).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   96.47%   96.52%   +0.04%     
==========================================
  Files          14       14              
  Lines         568      576       +8     
==========================================
+ Hits          548      556       +8     
  Misses         20       20              

Flag	Coverage Δ
Apex	96.52% <100.00%> (+0.04%)	⬆️

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