Name the pinned LiteRT-LM runtime tag in unsupported-runtime errors - #1920

Summary

• LiteRT-LM unsupported-runtime errors (litert_lm_service.dart, litert_lm_c_api_version.dart, litert_lm_runtime.dart) name the pinned tag from liteRtLmReleaseTag instead of the stale v0.19.1-2 (0.19.x audit follow-ups (index) #1861, Prepare 0.19.5 release #1917 audit). The "v0.19.1-2 or newer" floors in litert_lm_platform.dart are real minimums and stay.
• The tests now assert the pin, so they fail against the old text.

Production-readiness scope

• User-facing scope: Error text names the packaged runtime.
• Supported platforms/paths: Native LiteRT-LM (message text 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
• Targeted VM: test/unit/backends/litert_lm
• coverage-lib: message-only change; result in a PR comment

Matrix Evidence

Review Notes

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

High-risk regression review

• Classification: high-risk (backendRuntime)
• Implementation task: lane-1861 (0.19.x audit follow-ups (index) #1861)
• Independent blocking QA task: pending (draft; not requested)
• Exact head / current base: 9985b5c / edaf8b5
• Production-branch deletion, bypass, or miswire proof: message text only; tests assert the pin
• Affected-family real-model/artifact evidence: N/A: no model-dependent change; unit tests
• Explicit unavailable-family or other N/A evidence: N/A
• Known PR-caused P1 regressions: 0
• Unresolved review threads: 0