fix(chat): prevent panic on UTF-8 boundary when rendering response

[sandikodev]

Issue

Closes #3715

Problem

When the AI response contains multi-byte UTF-8 characters (e.g. non-ASCII text adjacent to triple backticks), the byte offset accumulated via parsed.offset_from() can land in the middle of a character boundary. The subsequent &buf[offset..] slice then panics at runtime:

The application panicked (crashed).
Message:  byte index 536 is out of bounds of `...`[...]
Location: crates/chat-cli/src/cli/chat/mod.rs:3849

The same pattern existed in two places:

• crates/chat-cli/src/cli/chat/mod.rs — streaming response rendering loop
• crates/chat-cli/src/cli/chat/parse.rs — validate! test macro loop (line 675)

Fix

Replace the direct slice with .get(offset..) which returns None instead of panicking when the index is out of bounds or misaligned, and break the loop gracefully.

// Before (both locations)
let input = Partial::new(&buf[offset..]);

// After
let Some(slice) = buf.get(offset..) else { break };
let input = Partial::new(slice);

Testing

# Run the regression test
cargo test -p chat_cli multibyte_utf8

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

The regression test covers three real-world inputs that triggered the panic:

• Indonesian text adjacent to triple backticks
• Chinese characters adjacent to triple backticks
• Emoji adjacent to triple backticks

Before this fix: the test panics with byte index N is out of bounds
After this fix: all inputs parse without panic

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[sandikodev]

Update (amended commit): The fix was originally applied only to mod.rs. On closer inspection, the identical &buf[offset..] pattern existed in parse.rs as well (inside the validate! test macro loop at line 675). Both locations are now fixed.

A regression test was also added in parse.rs with real-world inputs that triggered the panic in the wild — Indonesian text, Chinese characters, and emoji adjacent to triple backticks — to confirm the fix prevents the panic and catch any future regression.