Router: size the fit check from the decoded Data

[caveman99]

The receive-side fit check sized the raw wire Data, while the sender's signedDataFits() sizes the encoded struct. Fields skipped at decode make the two disagree. Size p->decoded on both sides so the gates stay symmetric.

note: this probably affects #10967

Summary by CodeRabbit

• Bug Fixes

  • Improved validation of signed and unsigned broadcasts to make downgrade decisions more accurate.
  • Packets containing unknown fields are now evaluated correctly without affecting decoded message data.
  • Unsigned broadcasts from recognized signers are properly rejected when they exceed the supported signed format size.
  • Valid broadcasts from non-signers continue to be accepted and delivered as expected.

• Tests

  • Added coverage for packet compatibility and signing-policy edge cases.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 527659a5-0f6a-4bb7-93df-c8c90b27d67a

📥 Commits

Reviewing files that changed from the base of the PR and between deea7be and bd4ca90.

📒 Files selected for processing (3)

• src/mesh/Router.cpp
• src/mesh/Router.h
• test/test_packet_signing/test_main.cpp

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📝 Walkthrough

Walkthrough

Changes

XEdDSA receive-policy flow

Layer / File(s)	Summary
Canonical downgrade sizing src/mesh/Router.h, src/mesh/Router.cpp	The policy API removes the external encoded-size argument and uses canonical protobuf re-encoding of p->decoded for unsigned-broadcast fit validation.
Pre-merge receive-policy validation src/mesh/Router.cpp	perhapsDecode applies the XEdDSA policy before merging the wire want_response bitfield.
Unknown-field regression coverage test/test_packet_signing/test_main.cpp	New helpers and A10/A11 tests cover encrypted packets with unknown fields, signer-based dropping, non-signer acceptance, and test registration; nearby comments are updated.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant perhapsDecode
  participant checkXeddsaReceivePolicy
  participant pb_get_encoded_size
  perhapsDecode->>checkXeddsaReceivePolicy: validate decoded packet
  checkXeddsaReceivePolicy->>pb_get_encoded_size: encode-size p->decoded
  pb_get_encoded_size-->>checkXeddsaReceivePolicy: canonical size
  checkXeddsaReceivePolicy-->>perhapsDecode: accept or drop
  perhapsDecode->>perhapsDecode: merge want_response bitfield

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Suggested reviewers: jp-bennett, thebentern, nomdetom

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is on-topic but largely incomplete versus the template; it lacks the required attestations and testing/regression details.	Replace the brief note with a fuller PR summary and complete the attestations/testing checklist, including devices tested and any regression notes.
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: sizing the router fit check from decoded Data.
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.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch decode-size

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