feat: add force_append_path parameter to http_options

[what-is-thy-bidding]

feat: Add force_append_path parameter to http_options for explicit URL control

Add new force_append_path parameter to http_options that gives users explicit
control over whether API paths are appended to custom base URLs. This solves
404 errors when using custom base URLs with API gateway proxies.

Problem:
When using custom base URLs (e.g., API gateway proxies) with Bearer token
authentication, the SDK wasn't appending model-specific API paths, resulting
in 404 errors. Users had to resort to workarounds like setting dummy api_key
values.

Solution:
The new force_append_path parameter provides three options:

• True: Always append API paths (recommended for custom proxies)
• False: Never append paths (for complete endpoint URLs)
• None: Use default SDK logic (backward compatible)

Changes:

• Added force_append_path: Optional[bool] field to HttpOptions (types.py)
• Added force_append_path: Optional[bool] field to HttpOptionsDict (types.py)
• Updated URL construction logic in _api_client.py to check force_append_path
• Priority: explicit force_append_path setting > default SDK logic
• Updated README.md with usage examples and documentation

Example usage:

client = genai.Client(
    vertexai=True,
    http_options={
        'base_url': 'https://my-proxy.com/ais/v1/',
        'force_append_path': True,  # Explicitly enable path appending
        'headers': {'Authorization': 'Bearer token'},
    },
)

Benefits:

• Clean, configuration-driven solution
• No workarounds or dummy values needed
• Backward compatible (None falls back to existing logic)
• Explicit user control over URL construction

Files modified:

• google/genai/types.py
• google/genai/_api_client.py
• README.md

Breaking changes: None
Backward compatible: Yes

[what-is-thy-bidding]

@sasha-gitg : can you please have a look at this solution to my 404 error?
I'd really appreciate some feedback, I am trying to learn as well :)

[janasangeetha]

Hi @what-is-thy-bidding
Thanks for your contributions!
The branch is out-of-date. Please update the same.

[what-is-thy-bidding]

@janasangeetha : Thanks for reaching back.
I have updated the branch and tested the changes again.

Please let me know incase more info is needed form me.
Thanks