Fix #2296 Add "minimal" reasoning effort for GPT 5 models (#2326)

This pull request resolves #2296; I've confirmed if it works by:

1. Add settings to ~/.codex/config.toml:
```toml
model_reasoning_effort = "minimal"
```

2. Run the CLI:
```
cd codex-rs
cargo build && RUST_LOG=trace cargo run --bin codex
/status
tail -f ~/.codex/log/codex-tui.log
```

Co-authored-by: pakrym-oai <pakrym@openai.com>

codex-rs/core/src/client_common.rs

@@ -94,6 +94,7 @@ pub(crate) struct Reasoning {
 #[derive(Debug, Serialize, Default, Clone, Copy)]
 #[serde(rename_all = "lowercase")]
 pub(crate) enum OpenAiReasoningEffort {
+    Minimal,
     Low,
     #[default]
     Medium,
@@ -103,6 +104,7 @@ pub(crate) enum OpenAiReasoningEffort {
 impl From<ReasoningEffortConfig> for Option<OpenAiReasoningEffort> {
     fn from(effort: ReasoningEffortConfig) -> Self {
         match effort {
+            ReasoningEffortConfig::Minimal => Some(OpenAiReasoningEffort::Minimal),
             ReasoningEffortConfig::Low => Some(OpenAiReasoningEffort::Low),
             ReasoningEffortConfig::Medium => Some(OpenAiReasoningEffort::Medium),
             ReasoningEffortConfig::High => Some(OpenAiReasoningEffort::High),

codex-rs/core/src/config_types.rs

@@ -206,6 +206,7 @@ impl From<ShellEnvironmentPolicyToml> for ShellEnvironmentPolicy {
 #[serde(rename_all = "lowercase")]
 #[strum(serialize_all = "lowercase")]
 pub enum ReasoningEffort {
+    Minimal,
     Low,
     #[default]
     Medium,