Fix RST rendering in extension READMEs

PyPI's strict description renderer rejected the previous markup, blocking
publish. Caught by a TestPyPI dry-run.

- dapr-ext-langgraph, dapr-ext-strands: title underline length matches title
- flask_dapr: replace markdown code fence with .. code-block:: python

Signed-off-by: Sergio Herrera <627709+seherv@users.noreply.github.com>

Author: seherv

ext/dapr-ext-langgraph/README.rst

@@ -1,5 +1,5 @@
 dapr-ext-langgraph extension
-=======================
+============================
 
 |pypi|
 

ext/dapr-ext-strands/README.rst

@@ -1,5 +1,5 @@
 dapr-ext-strands extension
-=======================
+==========================
 
 |pypi|
 

ext/flask_dapr/README.rst

@@ -20,17 +20,17 @@ Installation
 PubSub Events
 -------------
 
-```python
-from flask import Flask, request
-from flask_dapr import DaprApp
-
-app = Flask('myapp')
-dapr_app = DaprApp(app)
-@dapr_app.subscribe(pubsub='pubsub', topic='some_topic', route='/some_endpoint')
-def my_event_handler():
-  # request.data contains pubsub event
-  pass
-```
+.. code-block:: python
+
+    from flask import Flask, request
+    from flask_dapr import DaprApp
+
+    app = Flask('myapp')
+    dapr_app = DaprApp(app)
+    @dapr_app.subscribe(pubsub='pubsub', topic='some_topic', route='/some_endpoint')
+    def my_event_handler():
+      # request.data contains pubsub event
+      pass
 
 References
 ----------