docs: Update progressive SSE streaming to be enabled by default

- Change progressive SSE streaming from experimental to default-enabled
- Add documentation for function call argument streaming via partial_args
- Update environment variable usage to show how to disable (instead of enable)
- Add warning about trade-offs when using legacy behavior

Author: xuanyang15

docs/streaming/dev-guide/part4.md

@@ -223,26 +223,40 @@ sequenceDiagram
 
 ### Progressive SSE Streaming
 
-**Progressive SSE streaming** is an experimental feature that enhances how SSE mode delivers streaming responses. When enabled, this feature improves response aggregation by:
+**Progressive SSE streaming** is a feature that enhances how SSE mode delivers streaming responses. This feature improves response aggregation by:
 
 - **Content ordering preservation**: Maintains the original order of mixed content types (text, function calls, inline data)
 - **Intelligent text merging**: Only merges consecutive text parts of the same type (regular text vs thought text)
 - **Progressive delivery**: Marks all intermediate chunks as `partial=True`, with a single final aggregated response at the end
 - **Deferred function execution**: Skips executing function calls in partial events, only executing them in the final aggregated event to avoid duplicate executions
+- **Function call argument streaming**: Supports progressive building of function call arguments through `partial_args`, enabling real-time display of function call construction
 
-**Enabling the feature:**
+**Default Behavior:**
+
+Progressive SSE streaming is **enabled by default** in ADK. This means when you use `StreamingMode.SSE`, you automatically benefit from these improvements without any configuration.
+
+**Disabling the feature (if needed):**
 
-This is an experimental (WIP stage) feature disabled by default. Enable it via environment variable:
+If you need to revert to the legacy SSE streaming behavior (simple text accumulation), you can disable it via environment variable:
 
 ```bash
-export ADK_ENABLE_PROGRESSIVE_SSE_STREAMING=1
+export ADK_ENABLE_PROGRESSIVE_SSE_STREAMING=0
 ```
 
-**When to use:**
+!!! warning "Legacy Behavior Trade-offs"
+
+    Disabling progressive SSE streaming reverts to simple text accumulation, which:
+    - May lose original content ordering when mixing text and function calls
+    - Does not support function call argument streaming via `partial_args`
+    - May produce duplicate merged text events in addition to the final aggregated event
+    - Is provided for backward compatibility only—new applications should use the default progressive mode
+
+**When progressive SSE streaming helps:**
 
-- You're using `StreamingMode.SSE` and need better handling of mixed content types (text + function calls)
+- You're using `StreamingMode.SSE` and have mixed content types (text + function calls)
 - Your responses include thought text (extended thinking) mixed with regular text
 - You want to ensure function calls execute only once after complete response aggregation
+- You need to display function call construction in real-time as arguments stream in
 
 **Note:** This feature only affects `StreamingMode.SSE`. It does not apply to `StreamingMode.BIDI` (the focus of this guide), which uses the Live API's native bidirectional protocol.