Add Area-based Pool logic for regional URL cycling

[fern-support]

Summary

Implements Area-based Pool logic for regional URL cycling in the Python SDK, mirroring the implementation from the Go SDK PR.

Key additions:

• Area enum (US, EU, AP, CN) representing global regions
• Pool class for managing regional URLs with automatic cycling and DNS-based domain selection
• Both sync (select_best_domain) and async (select_best_domain_async) DNS resolution
• Support for both agora.io and sd-rtn.com domain suffixes
• next_region() for cycling through region prefixes on failure
• Agora and AsyncAgora clients now include domain pool integration

Updates since last revision

Refactored to extend base classes instead of duplicating code:

• Agora now extends the base Agora class with domain pool support
• AsyncAgora now extends the base AsyncAgora class with domain pool support
• Only the pool-enabled clients are exported from the main package (original exports replaced)
• Added edited files to .fernignore to prevent Fern from overwriting custom code

Usage: from agoraio import Agora, AsyncAgora, Area

Review & Testing Checklist for Human

• Export change: The original Agora/AsyncAgora exports are replaced with pool-enabled versions. Verify this doesn't break existing consumers who don't want pool functionality (they must now provide area parameter)
• Private attribute access: The clients directly modify _client_wrapper._base_url to update the base URL. Verify this internal access pattern is acceptable.
• Thread safety in Pool class: The _domain_need_update() check happens outside the lock before acquiring it in select_best_domain(). Verify this double-checked locking pattern is acceptable.
• Async lock usage: select_best_domain_async uses a synchronous threading.Lock inside an async method, which could block the event loop. Consider if this should use asyncio.Lock instead.

Test plan:

from agoraio import Agora, Area

client = Agora(
    area=Area.US,
    username="YOUR_USERNAME",
    password="YOUR_PASSWORD",
)
print(client.get_current_url())  # Should print https://api-us-west-1.agora.io
client.next_region()
print(client.get_current_url())  # Should print https://api-us-east-1.agora.io

# Verify inherited properties work
print(client.agents)  # Should return AgentsClient instance

Notes

• No unit tests are included in this PR - tests should be added separately
• This is part of a coordinated change across Go, TypeScript, and Python SDKs

Link to Devin run: https://app.devin.ai/sessions/ece6fb9b8ad84ec68720a1bcd7d64f93
Requested by: @fern-support

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