Copybara import of the project:

--
ad923c2c8c503ba73c62db695e88f1a3ea1aeeea by YU MING HSU <abego452@gmail.com>:

docs: enhance Contribution process within CONTRIBUTING.md

--
8022924fb7e975ac278d38fce3b5fd593d874536 by YU MING HSU <abego452@gmail.com>:

fix: move _maybe_append_user_content from google_llm.py to base_llm.py,
so subclass can get benefit from it, call _maybe_append_user_content
from generate_content_async within lite_llm.py

--
cf891fb1a3bbccaaf9d0055b23f614ce52449977 by YU MING HSU <abego452@gmail.com>:

fix: modify install dependencies cmd, and use pyink to format codebase
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/428 from hsuyuming:fix_litellm_error_issue_427 dbec4949798e6399a0410d1b8ba7cc6a7cad7bdd
PiperOrigin-RevId: 754124679

CONTRIBUTING.md

@@ -24,6 +24,35 @@ This project follows
 [Google's Open Source Community Guidelines](https://opensource.google/conduct/).
 
 ## Contribution process
+1.  **Clone the repository:**
+
+    ```shell
+    git clone git@github.com:google/adk-python.git
+    cd adk-python
+    ```
+2.  **Create and activate a virtual environment:**
+
+    ```shell
+    python -m venv .venv
+    source .venv/bin/activate
+    pip install uv
+    ```
+
+3.  **Install dependencies:**
+
+    ```shell
+    uv pip install -e .[dev,test,extensions,eval]
+    ```
+4.  **Run unit tests:**
+
+    ```shell
+    uv run pytest ./tests/unittests
+    ```
+5.  **Run pyink to format codebase:**
+
+    ```shell
+    uv run pyink  --config pyproject.toml ./src
+    ```
 
 ### Requirement for PRs