Fix case-insensitive request header merging

[andreitava-uip]

Summary

• collapse differently-cased request header fields in the shared sync and async HTTPX clients
• use the casing from the final occurrence while preserving repeated values that intentionally use that spelling
• normalize before HTTPX authentication so custom auth signs the same header set sent to transport

Root cause

Provider SDKs can compose headers through case-sensitive dictionaries. Anthropic added User-Agent while the shared HTTPX client supplied user-agent, resulting in two raw fields for the same case-insensitive HTTP header. The HTTP.sys endpoint rejected that request with HTTP 400 Bad Request - Invalid Header even though each field name and value was valid independently.

Verification

• uv run pytest tests/core/features/test_httpx_client.py -q — 31 passed
• uv run pytest tests/core/features -q — 257 passed
• Ruff lint and format checks passed
• reviewer re-reviewed after two fixes and reported CLEAN
• live claude-opus-4-6 request succeeded with the playground workaround disabled and one final User-Agent field