fix: set timeout to 895 seconds

[ionut-mihalache-uipath]

Summary

Raises the default request timeout for LLM/chat clients from 300s → 895s, just under the 15-minute API-gateway ceiling. Long-running model calls (large prompts, deep reasoning, tool-heavy agents) were getting cut off at 5 minutes; this gives them headroom while staying inside the upstream limit.

Changes

• src/uipath_langchain/_utils/_environment.py — get_default_timeout() fallback for UIPATH_TIMEOUT_SECONDS is now 895 (was 300).
• src/uipath_langchain/_utils/_settings.py — UiPathClientSettings.timeout_seconds default is now "895" (was "300").
• src/uipath_langchain/chat/chat_model_factory.py — DEFAULT_TIMEOUT_SECONDS constant and docstring updated to 895.0.
• src/uipath_langchain/chat/_legacy/bedrock.py — AwsBedrockCompletionsPassthroughClient no longer hardcodes read_timeout=300; it now calls get_default_timeout() so Bedrock honors the same env-driven value as
  the other providers.
• pyproject.toml / uv.lock — version bump 0.10.20 → 0.10.21.

Behavior

• Callers that don't pass an explicit timeout now get 895s.
• Callers that set UIPATH_TIMEOUT_SECONDS are unaffected (env var still wins).
• Bedrock passthrough now respects UIPATH_TIMEOUT_SECONDS instead of being pinned at 300s.