fix(llm): send Harness session headers through pi-ai

[fzfz]

OpenCode Go rejects model calls with 400 MissingSessionID because the pi-ai adapter passes sessionId to the SDK without emitting a session header. Add the Harness-native x-deepseek-harness-session-id header from each call's session ID, matching the DeepSeek adapter and the header OpenCode Go recognizes.

The current session ID overrides any configured value for x-deepseek-harness-session-id, regardless of header-name casing. Unrelated request headers remain intact, and calls without a session ID do not receive a generated ID.

Validation:

• All nine session-header and provider-error tests passed on this upstream branch. The seven session-header tests exercise the real PiAiAdapter across OpenAI Completions, Anthropic Messages, and OpenAI Responses.
• Tests cover repeated/prepared calls, concurrent parent/child sessions, numeric IDs, conflicting configured session headers, and missing IDs.
• Type checking and git diff --check passed; the patch reverse-application check passed against the installed package.
• The upstream full suite passed 718 tests; two PPT suites could not load because the reused development dependencies do not contain dsh-ppt. On the fork baseline, the full suite passed all 724 tests.
• A real OpenCode Go deepseek-v4-flash request returned OK with a successful stop after applying the patch.

Provider requirements: https://opencode.ai/docs/go/#where-can-i-use-it