[FEATURE] Allow opt-in to line-by-line streaming on WSL (override WT_SESSION detection)

v2.1.81 disabled line-by-line streaming on WSL in Windows Terminal. The detection uses WT_SESSION and WT_PROFILE_ID env vars.

For users whose terminals handle streaming correctly (e.g., WSL2 with Windows Terminal builds that have fixed the original rendering bugs), there is currently no way to re-enable it.

Request: An env var override like CLAUDE_CODE_FORCE_STREAMING=1 to opt back in to line-by-line streaming regardless of terminal detection.

Context:

• #36862 documents the missing docs for this change
• #37110 reports it as a regression
• Neither requests an override mechanism

The detection is overly broad — it disables streaming for all WSL users in Windows Terminal, even those with terminal versions where the rendering issues are fixed.