bug: ValidationException "Improperly formed request" after long conversation + str_replace

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

wsl linux ubuntu

### Expected behaviour

Description: After a lengthy conversation (with context summary loaded), attempting a str_replace edit triggers two
consecutive ValidationException errors with "Improperly formed request" at crates/chat-cli/src/cli/chat/mod.rs:1467. The
edit actually succeeds but the response fails. No user-friendly recovery message — just raw Rust backtraces. Should
suggest /compact instead.

### Actual behaviour

Description: After a lengthy conversation (with context summary loaded), attempting a str_replace edit triggers two
consecutive ValidationException errors with "Improperly formed request" at crates/chat-cli/src/cli/chat/mod.rs:1467. The
edit actually succeeds but the response fails. No user-friendly recovery message — just raw Rust backtraces. Should
suggest /compact instead.

### Steps to reproduce

fill a context and keep going with a query

### Environment

```yaml

```