docs: Fix Helloworld Quick Start and document uv non-project alternative

[Iwaniukooo11]

Description

This PR resolves issue #635 where the root README Quick Start for the Python helloworld sample was conflicting with uv's parent project traversal.

Since helloworld does not contain a pyproject.toml file (by design, to favor standard requirements.txt / pip environments), running uv run inside the folder triggers resolution using the parent project's samples/python/pyproject.toml and locks the environment, leading to a ModuleNotFoundError on startup.

Changes

1. Root README.md:
   • Updated Quick Start instructions to guide users to create a virtual environment (python -m venv .venv) and install dependencies using standard pip (pip install -r requirements.txt).
2. samples/python/README.md:
   • Re-organized the main running instructions into distinct options: Option A (standard pip + virtualenv) and Option B (uv for pyproject.toml projects).
   • Added an alternative tips section explaining how to run standard requirements.txt samples with uv using non-project mode (uv venv and uv pip install).
   • Cleaned up trailing whitespace and ensured standard newline endings.

Verification

• Verified both README files are clean of trailing whitespace.
• Verified that running python -m venv .venv && source .venv/bin/activate && pip install -r requirements.txt builds the virtual environment with the correct SDK version (a2a-sdk>=1.1.0) and starts the server successfully.

[gemini-code-assist]

Code Review

This pull request updates the README documentation to provide alternative setup and execution instructions using standard pip and virtualenv (Option A) alongside the existing uv-based instructions (Option B). The review feedback points out that the instructions for running the host/test clients are missing in both options, and suggests adding these steps to ensure the guides are complete and actionable.

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