Refuse unparseable and uppercase-scheme model URLs in the JSON Schema device e2e - #1921

Summary

• chat-app-litert-json-schema e2e: refuse an http(s):// --model-path that Uri.tryParse cannot parse (it failed open; the value was echoed and compiled into a dart-define), and detect http(s) case-insensitively so HTTPS://host/m is a URL, not a local path (0.19.x audit follow-ups (index) #1861, Gate native LiteRT-LM JSON Schema per qualified host and accelerator #1909 re-audit).
• The new cases fail against origin/main; making the scheme match case-sensitive again fails the HTTPS:// case.

Production-readiness scope

• User-facing scope: None (maintainer e2e tooling).
• Supported platforms/paths: tool/testing only.
• Unsupported or intentionally unavailable paths: None changed.
• Out of scope / follow-ups: Remaining 0.19.x audit follow-ups (index) #1861 items are tracked there.

Completeness checklist

• Declared scope is fully implemented.
• Unsupported platform/option combinations: unchanged.
• Changelog entries updated where user-visible (N/A for tooling-only).
• Regression coverage covers the original finding and fails first against origin/main.
• Validation coverage registry: N/A, no capability change.
• No secrets or signed URLs leak through logs or errors.
• Follow-ups: None beyond 0.19.x audit follow-ups (index) #1861.

PR type guidance

• Bugfix PR: root cause and regression test are in the summary.

Test Plan

• dart run tool/prepare_workspace.dart
• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm test/unit/tooling/: only the six windows_registry_qualification_test.dart cases fail locally (no pwsh)

Matrix Evidence

Review Notes

• Independent review status: N/A (not yet requested)
• CI status / head SHA: pending

High-risk regression review

• Classification: standard
• Implementation task: N/A
• Independent blocking QA task: N/A
• Exact head / current base: N/A
• Production-branch deletion, bypass, or miswire proof: N/A
• Affected-family real-model/artifact evidence: N/A
• Explicit unavailable-family or other N/A evidence: N/A
• Known PR-caused P1 regressions: 0
• Unresolved review threads: 0