feat: auto-continue after server overload

[AVas112]

What changed

• The TUI now automatically submits Continue after a ServerOverloaded error.
• Auto-continuation runs for every overload event and is not blocked by the quota pause state.
• Recovery state for ServerOverloaded is kept separate from UsageLimitExceeded, preserving the existing usage-limit behavior.
• Duplicate Continue entries are prevented by deduplicating the locally rendered prompt and its app-server echo using both turn_id and message content.
• The same echo deduplication behavior is preserved for safety-buffering retries.
• Added a configuration toggle for enabling or disabling overload auto-continuation.

Why

When the selected model is overloaded, the current turn ends with Selected model is at capacity. Previously, the user had to submit a continuation manually. Codext now starts the next turn automatically so work can continue without manual intervention.

Configuration

Auto-continuation is enabled by default. To disable it, add the following to config.toml:

[tui]
server_overloaded_resume_enabled = false

Validation

• cargo build -p codex-cli
• cargo build --release -p codex-cli
• git diff --check
• Project linters
• Architecture review completed with no findings
• Configuration schema regenerated

Tests and snapshot files were not changed, in accordance with the temporary rust-v0.145.0 reapply guardrails.