Support Python dependencies in Server

[jlowin]

When developing and deploying a server, we want to be able to specify Python dependencies without having to do --with dep every time.

In earlier versions of FastMCP, it was possible to use uv's [script dependencies] (https://docs.astral.sh/uv/guides/scripts/#declaring-script-dependencies) to declare dependenceis as frontmatter, but since we now use fastmcp run server.py instead of uv run server.py by default, they don't get parsed.

This PR makes the following code possible (which requires pandas) while keeping the install command fastmcp install demo.py

# demo.py

from fastmcp import FastMCP

mcp = FastMCP("demo", dependencies=['pandas']

@mcp.tool()
def demo():
    import pandas
    ...

The README is also updated to clarify that:

• fastmcp install will respect server dependencies when creating an environment for Claude desktop
• fastmcp dev will respect server dependencies when creating an environment for the MCP Inspector
• fastmcp run will NOT respect dependencies; it assumes they are already set up. However it does not require a if name == 'main' block
• python run and uv run are perfectly valid as long as the user calls mcp.run() themselves and takes responsibility for all dependencies.