perf: Eliminate unnecessary string allocations in config handling

Start in crates/terraphim_service/src/lib.rs at lines 86-89, 195-198, 342-348, 421-428, and 527-532, then trace callers of the configuration helpers. Review the performance analysis from PR #429 and check all affected return types. Done means the unnecessary clones and allocations are removed while configuration handling retains its behavior and the expected allocation improvement is verified.

Issue Description

Unnecessary string cloning and allocations in configuration lookups add GC pressure.

Location

crates/terraphim_service/src/lib.rs (lines 86-89, 195-198, 342-348, 421-428, 527-532)

Current Code

```rust
// Line 86-89: Cloning entire HashMap
let nested_map: AHashMap<String, Value> = nested_obj
.iter()
.map(|(k, v)| (k.clone(), v.clone()))
.collect();

// Line 195-198: Unnecessary to_string()
fn get_string_extra(extra: &AHashMap<String, Value>, key: &str) -> Option {
extra.get(key).and_then(|v| v.as_str().map(|s| s.to_string()))
// ^^^^^^^^^^^^^^^^ Unnecessary allocation
}
```

Impact

• MEDIUM priority - Performance
• Adds GC pressure for frequent calls
• Unnecessary memory allocations

Recommended Fix

```rust
// Return Cow to avoid allocation when possible
fn get_string_extra<'a>(
extra: &'a AHashMap<String, Value>,
key: &str
) -> Option<Cow<'a, str>> {
extra.get(key).and_then(|v| {
v.as_str().map(|s| Cow::Borrowed(s))
})
}

// For nested_map, use references instead
fn build_from_nested_extra(
nested_obj: &serde_json::Map<String, Value>
) -> Option<Arc> {
// Use get_string_extra directly on nested_obj
if let Some(provider) = nested_obj.get("llm_provider")
.and_then(|v| v.as_str())
{
// ...
}
}
```

Expected Improvement

• 20-30% reduction in allocations for config lookups
• Reduced memory pressure
• Faster config operations

References

• Identified in performance analysis for PR #429
• Related to configuration handling