fix(todo): improve validation error messages to help model self-correct

[sandikodev]

Issue

Closes #3184

Problem

When the model sends invalid todo_list parameters for large todo lists, the validation error messages were too terse for the model to self-correct:

Tool validation failed: Insertion indices must be unique
Tool validation failed: Must provide an index for every new task
Tool validation failed: Index 44 is out of bounds for length 43,

These messages don't give the model enough context to understand what correction is needed — especially the count mismatch and bounds errors.

Fix

Improve all three validation error messages with actionable context:

Before	After
Insertion indices must be unique	Insertion indices must be unique — each task must have a distinct index
Must provide an index for every new task	Must provide an index for every new task — got 5 tasks but 3 indices
Index 44 is out of bounds for length 43,	Index 44 is out of bounds — valid insertion indices are 0..=43 (current list has 43 tasks)

The same improvement is applied to Complete and Remove operations.

Testing

# Run the full test suite to check for regressions
cargo test -p chat_cli

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