feat(error): add CantDoReason::InvalidPayload

[grunch]

Consumed by mostro's Phase 2 anti-abuse-bond admin handlers, per docs/ANTI_ABUSE_BOND.md §7.3 / §7.5.

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Summary by CodeRabbit

• New Features
  • Added improved error handling for invalid or unprocessable request payloads, providing clearer feedback when payload structures are mismatched or cannot be processed as expected.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: cd01279f-9f9c-4455-8396-335ce1380686

📥 Commits

Reviewing files that changed from the base of the PR and between 75b4523 and 14096f7.

📒 Files selected for processing (1)

• src/error.rs

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Walkthrough

This PR adds a new InvalidPayload variant to the CantDoReason error enum in src/error.rs. The variant describes mismatched or unprocessable request payloads. A unit test verifies that the variant correctly serializes to "invalid_payload" in JSON and deserializes back to the same enum value.

Changes

CantDoReason Error Enum Extension

Layer / File(s)	Summary
InvalidPayload variant and serialization test src/error.rs	CantDoReason::InvalidPayload is added with documentation for invalid or unprocessable request payloads, and a unit test confirms JSON serialization round-trips using snake_case naming.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• MostroP2P/mostro-core#118: Both PRs extend the CantDoReason enum with new variants and affect the error taxonomy at the API level.
• MostroP2P/mostro-core#76: Both PRs add variants to CantDoReason and update serialization tests for the error enum.

Suggested reviewers

• Catrya

Poem

A payload arrives, twisted and strange,
InvalidPayload's here to rearrange,
With snake_case precision and tests that ring true,
The error enum grows—another path through! 🐰✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding a new enum variant InvalidPayload to the CantDoReason error type. It is concise, specific, and clearly communicates the primary change to the changeset.
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.

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✨ 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 feat/error-invalid-payload

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