Set cache breakpoint on second-to-last message (#27632)
Here's a sample `dbg!` of token usage after this change, for a small
agent thread:
```
[crates/assistant2/src/thread.rs:1092:25] &usage = TokenUsage {
input_tokens: 5354,
output_tokens: 184,
cache_creation_input_tokens: 0,
cache_read_input_tokens: 0,
}
[crates/assistant2/src/thread.rs:1092:25] &usage = TokenUsage {
input_tokens: 54,
output_tokens: 132,
cache_creation_input_tokens: 5518,
cache_read_input_tokens: 0,
}
[crates/assistant2/src/thread.rs:1092:25] &usage = TokenUsage {
input_tokens: 54,
output_tokens: 113,
cache_creation_input_tokens: 166,
cache_read_input_tokens: 5518,
}
[crates/assistant2/src/thread.rs:1092:25] &usage = TokenUsage {
input_tokens: 291,
output_tokens: 181,
cache_creation_input_tokens: 147,
cache_read_input_tokens: 5684,
}
```
Release Notes:
- N/A
edc7d73643