fix(acp): guard vim.NIL in tool-call message builder

[carlos-algms]

• providers send JSON null for tool-call fields
• vim.NIL is truthy userdata, broke ipairs/index
• crashed on session restore via __build_tool_call_message
• guards content, locations, rawInput

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 5d17650b-f399-4e8b-a238-488f4513d885

📥 Commits

Reviewing files that changed from the base of the PR and between 326682f and fc0b9af.

📒 Files selected for processing (3)

• .agents/skills/agentic-acp-protocol-flow/references/transport-framing.md
• lua/agentic/acp/acp_transport.lua
• lua/agentic/acp/acp_transport.test.lua

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📝 Walkthrough

Walkthrough

This PR tightens ACPClient:__build_tool_call_message to only treat update.content, update.rawInput, and update.locations as iterables or fallback sources when they are explicitly tables. It also adds ACPTransport.decode_line — a wrapper around vim.json.decode using luanil so JSON null maps to Lua nil — replaces inline decoding in the stdout loop with this helper, and adds tests and docs verifying null→nil decoding and malformed JSON handling.

Sequence Diagram(s)

sequenceDiagram
  participant StdoutReader
  participant ACPTransport
  participant vim_json_decode
  participant Callbacks
  StdoutReader->>ACPTransport: send non-empty line
  ACPTransport->>vim_json_decode: decode(line, { luanil = { object = true, array = true } })
  vim_json_decode-->>ACPTransport: (ok, decoded)  Note right of ACPTransport: JSON null -> Lua nil
  ACPTransport->>Callbacks: callbacks.on_message(ok, decoded)

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Possibly related PRs

• carlos-algms/agentic.nvim#162: Also modifies __build_tool_call_message logic and adapter-related tool-call construction.
• carlos-algms/agentic.nvim#79: Related changes to extraction of update.rawInput/content/locations in tool-call construction and tests.

Poem

🐰 I hop through lines of code with glee,
Nulls become nils for you and me,
Tables-only gates now stand up tall,
Errors flee and tests recall,
A rabbit cheers the safer call.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% 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
Title check	✅ Passed	The title directly addresses the main change: adding guards for vim.NIL in the tool-call message builder to fix crashes.
Description check	✅ Passed	The description relates to the changeset by explaining the problem (vim.NIL causing crashes) and the solution (guarding content, locations, rawInput fields).
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 unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/acp-tool-call-vim-nil-userdata

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