Fix multi-target fire-and-forget payload lifetime

[pcnudde]

Fixes #5291.

A multi-target streamed fire-and-forget send could retire its downloadable payload after the first receiver finished, leaving later recipients unable to download it. Declare the distinct receiver count before encoding and avoid creating a source for an empty target list.

Keep completion count-based: receivers can enable pass-through locally, so routing targets are not necessarily the final consumers. Missing recipients retain the source until existing timeout handling settles them; this change does not add retries or exact recipient-identity verification.

Validation:

• 119 focused Cell/downloader tests passed, including delayed/missing receivers and actual Adapter channel/topic forwarding.
• Separate-process TCP with an 8 MiB NumPy payload and controlled receiver scheduling passed; this is not a claim of a spontaneous production incident.
• ./runtest.sh -s passed for Python/style targets; the tool skipped notebook formatting because Jupyter extras were unavailable. No notebooks changed.
• Full repository tests and TLS deployment validation were not run.

[pcnudde]

/build

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; no actionable regressions were identified.

Summary

• Uses count-based completion so receiver-configured pass-through can finish under downstream consumer identities.
• Returns an empty result and FUTURES mapping without allocating a downloadable source when there are no targets.
• Adds regression coverage for delayed and missing receivers, duplicate and self targets, shared-context preservation, and actual Adapter channel/topic forwarding.
• No actionable issues were identified. Tests were inspected but not executed during this review.

Diagram

sequenceDiagram
    participant S as Sender Cell
    participant D as Download source
    participant A as Consumer A
    participant B as Consumer B
    S->>S: Count distinct targets
    S->>D: Encode with receiver count 2
    S-->>A: Send payload references
    S-->>B: Send payload references
    A->>D: Download and complete
    Note over D: Retain payload for remaining consumer
    alt Second consumer completes
        B->>D: Download and complete
        D->>D: Release payload with completed outcome
    else Second consumer is missing
        D->>D: Existing timeout settles and releases payload
    end

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.25%. Comparing base (83ec31a) to head (92e20e8).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5297      +/-   ##
==========================================
+ Coverage   66.87%   67.25%   +0.38%     
==========================================
  Files        1021     1021              
  Lines      106100   106129      +29     
==========================================
+ Hits        70951    71377     +426     
+ Misses      35149    34752     -397     

Flag	Coverage Δ
unit-tests	67.25% <100.00%> (+0.38%)	⬆️

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