Dockerfile.test.ts splitIntoStages mis-parses FROM lines with --platform flag

[itsmiso-ai]

Ask: Fix the splitIntoStages helper in Dockerfile.test.ts so a FROM --platform=... base AS name line is parsed into stage name mapping to the correct base image.

Found while reviewing PR #1175 (pre-existing, not introduced there; that PR's new MCP-wiring tests depend on this helper).

The regex used by splitIntoStages in Dockerfile.test.ts captures the first token after FROM as the base image, so a line like FROM --platform=$BUILDPLATFORM base AS builder would capture --platform=$BUILDPLATFORM / mis-associate the stage name. The current Dockerfile happens to have no --platform usage, so the bug is latent, but any future multi-platform stage would silently produce a wrong stage map and give false pass/fail results across every stage-scoped assertion in the file.

Suggested fix: strip leading --platform=... (and any other -- flags) before capturing the image and optional AS name, and add a unit test covering the FROM --platform=linux/amd64 node:24 AS foo shape.

Expected file to touch: Dockerfile.test.ts.

[itsmiso-ai]

Grooming could not verify the premise because Dockerfile.test.ts was only surfaced as an unpinned search result, not read at the pinned revision. A current read of the splitIntoStages helper is needed before this can be marked ready.