Add support for custom session/client injection for proxy authentication

[Barneyjm]

Summary

Adds the ability to pass a custom requests.Session (sync) or httpx.AsyncClient (async) to Tavily clients, enabling integration with authentication proxies without distributing API keys to each application team.

Motivation

When using Tavily behind an authentication proxy (e.g., client → some API Management → Tavily API), organizations need to:

• Centralize API key management at the proxy level
• Leverage existing proxy authentication mechanisms (subscription keys, OAuth, etc.)
• Avoid distributing Tavily API keys to individual application teams

Changes

• TavilyClient: Added optional session parameter to accept a pre-configured requests.Session
• AsyncTavilyClient: Added optional client parameter to accept a pre-configured httpx.AsyncClient
• Both implementations properly merge custom headers and proxy settings
• Maintains backward compatibility - creates default session/client if none provided

Usage Example

import requests
from tavily import TavilyClient

# Create custom session with proxy auth
custom_session = requests.Session()
custom_session.headers.update({
    "custom-header": "your-key"
})

# Use with Tavily client
client = TavilyClient(
    api_key="dummy-key",  # Can be placeholder when using proxy auth
    api_base_url="https://your-api.net/tavily",
    session=custom_session
)

response = client.search("query")

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes how `TavilyClient` creates/manages HTTP sessions and merges headers/proxies, which can affect request authentication/routing for existing integrations. Behavior is covered by new tests but could surface subtle precedence/regression issues in real network environments.
> 
> **Overview**
> **Adds support for injecting a custom `requests.Session` into `TavilyClient`.** The client now merges default Tavily headers and proxy settings into the provided session *without overriding* any existing session `headers` (including `Authorization`) or `proxies`, and tracks whether the session is external.
> 
> `close()`/context manager cleanup is updated to only close internally-created sessions, and `self.headers` is now kept as Tavily-specific defaults rather than mirroring the full session headers. A new `tests/test_custom_session.py` suite exercises header/proxy precedence and lifecycle behavior.
> 
> <sup>Written by [Cursor Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit 0a86fd8dcbd1303d1c2fc764c73470ee2176d6ef. This will update automatically on new commits. Configure [here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

[cursor]

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[Barneyjm]

this PR also allows for the functionality requested in #128 and allows more flexibility on top.