[BUG] ask_followup_question fails with "missing required parameter follow_up"

[awschmeder]

Description

The ask_followup_question tool intermittently fails with the error
missing required parameter follow_up, even though the model did supply a
follow_up value. The failure occurs when a model emits follow_up as a single
bare object (one suggestion) instead of wrapping it in an array:

// What the model sometimes emits (rejected):
{ "question": "Proceed?", "follow_up": { "text": "Yes", "mode": null } }

// What the tool requires (accepted):
{ "question": "Proceed?", "follow_up": [{ "text": "Yes", "mode": null }] }

The tool's JSON schema (src/core/prompts/tools/native-tools/ask_followup_question.ts)
declares follow_up as type: "array" with minItems: 1 -- so a single
suggestion is valid. The problem is the missing array wrapper, not the number
of suggestions. The tool description text ("a list of 2-4 suggested answers")
likely nudges some models into thinking a single answer does not need an array.

Root Cause

There are two parser surfaces, but only one is responsible for the runtime error:

• NativeToolCallParser.parseToolCall (line ~825) -- the finalized tool call
  path that produces the nativeArgs actually passed to tool execution. It sets
  follow_up: args.follow_up with no coercion, so a bare object passes through
  unchanged.
• AskFollowupQuestionTool.execute (line ~38) -- guards with
  !follow_up || !Array.isArray(follow_up) and, on failure, calls
  recordMissingParamError("follow_up"). Because the value is a non-array object,
  this guard fires and produces the misleading "missing parameter" message even
  though the parameter was present.

The streaming/partial path
(NativeToolCallParser.createPartialToolUse, line ~480) also drops a non-array
follow_up to undefined, but its output is only consumed by
AskFollowupQuestionTool.handlePartial, which reads question only and never
touches follow_up. It is therefore not a cause of the observed error, only
a cosmetic inconsistency in partial UI rendering.

Affected Files

• src/core/assistant-message/NativeToolCallParser.ts (lines ~480, ~825)
• src/core/tools/AskFollowupQuestionTool.ts (lines ~38, ~46)

Proposed Fix

1. In NativeToolCallParser.parseToolCall, coerce a bare-object follow_up into a
   single-element array before assigning nativeArgs (check Array.isArray first
   so an existing array is never re-wrapped).
2. Apply the same coercion in createPartialToolUse for partial-render consistency.
3. Correct the misleading error: when follow_up is present but not an array,
   report a malformed-parameter error rather than "missing required parameter".
4. (Optional robustness) Filter out suggestions lacking a text field so a
   malformed single suggestion fails clearly rather than producing an empty answer.

Test Procedure

1. Add a unit test for NativeToolCallParser.parseToolCall asserting that a bare
   object follow_up is coerced to a single-element array in nativeArgs.
2. Add a unit test for AskFollowupQuestionTool.execute asserting that a single
   valid suggestion (array of one) succeeds and reaches task.ask.
3. Verify that an existing multi-suggestion array still works unchanged.
4. Manual: trigger a follow-up question in F5 development mode and confirm a
   single-suggestion response renders and is accepted.

[awschmeder]

See #695 -- likely root cause after further investigation - not related to tool use error or instructions