Refactor: deduplicate JSON parsing, provider constructors, and identity function

Agent: fowler

Eliminate code-path aliasing and near-duplicates across recent commits:

1. Deduplicate find_json_object_end: Three near-identical copies in
   streaming_parser.rs, context_window.rs, and acd.rs consolidated into
   a single canonical implementation in utils.rs. All callers now route
   through the canonical version. The utils.rs version uses the most
   defensive variant (with found_start guard). (-84 lines)

2. Deduplicate provider constructors: AnthropicProvider::new() and
   GeminiProvider::new() now delegate to their respective new_with_name()
   methods instead of duplicating the full constructor body.
   (OpenAI already delegated.) (-28 lines)

3. Inline convert_cache_control: Removed identity function that just
   cloned CacheControl. Call sites now use .map(|cc| cc.clone())
   directly. (-4 lines)

Net: -65 lines, 0 behavior changes, all 683 library tests pass.

crates/g3-providers/src/anthropic.rs

@@ -145,26 +145,7 @@ impl AnthropicProvider {
         enable_1m_context: Option<bool>,
         thinking_budget_tokens: Option<u32>,
     ) -> Result<Self> {
-        let client = Client::builder()
-            .timeout(Duration::from_secs(300))
-            .build()
-            .map_err(|e| anyhow!("Failed to create HTTP client: {}", e))?;
-
-        let model = model.unwrap_or_else(|| "claude-3-5-sonnet-20241022".to_string());
-
-        debug!("Initialized Anthropic provider with model: {}", model);
-
-        Ok(Self {
-            client,
-            name: "anthropic".to_string(),
-            api_key,
-            model,
-            max_tokens: max_tokens.unwrap_or(32768),
-            temperature: temperature.unwrap_or(0.1),
-            cache_config,
-            enable_1m_context: enable_1m_context.unwrap_or(false),
-            thinking_budget_tokens,
-        })
+        Self::new_with_name("anthropic".to_string(), api_key, model, max_tokens, temperature, cache_config, enable_1m_context, thinking_budget_tokens)
     }
 
     /// Create a new AnthropicProvider with a custom name (e.g., "anthropic.default")
@@ -222,10 +203,7 @@ impl AnthropicProvider {
         builder
     }
 
-    fn convert_cache_control(cache_control: &crate::CacheControl) -> crate::CacheControl {
-        // Anthropic uses the same format, so just clone it
-        cache_control.clone()
-    }
+    // Anthropic uses the same CacheControl format — no conversion needed, just clone at call sites.
 
     fn convert_tools(&self, tools: &[Tool]) -> Vec<AnthropicTool> {
         tools
@@ -312,7 +290,7 @@ impl AnthropicProvider {
                             cache_control: message
                                 .cache_control
                                 .as_ref()
-                                .map(Self::convert_cache_control),
+                                .map(|cc| cc.clone()),
                         });
                     } else {
                         // Regular user message: images as top-level blocks, then text
@@ -330,7 +308,7 @@ impl AnthropicProvider {
                             cache_control: message
                                 .cache_control
                                 .as_ref()
-                                .map(Self::convert_cache_control),
+                                .map(|cc| cc.clone()),
                         });
                     }
 
@@ -349,7 +327,7 @@ impl AnthropicProvider {
                             cache_control: message
                                 .cache_control
                                 .as_ref()
-                                .map(Self::convert_cache_control),
+                                .map(|cc| cc.clone()),
                         });
                     }
 

crates/g3-providers/src/gemini.rs

@@ -77,14 +77,7 @@ impl GeminiProvider {
         max_tokens: Option<u32>,
         temperature: Option<f32>,
     ) -> Result<Self> {
-        Ok(Self {
-            client: Client::new(),
-            api_key,
-            model: model.unwrap_or_else(|| "gemini-2.0-flash".to_string()),
-            max_tokens: max_tokens.unwrap_or(16384),
-            temperature: temperature.unwrap_or(0.1),
-            name: "gemini".to_string(),
-        })
+        Self::new_with_name("gemini".to_string(), api_key, model, max_tokens, temperature)
     }
 
     pub fn new_with_name(