Phase 3b: raise the schema floor to 0.6.0-alpha, retire 0.4.0/0.5.0

[MGudgin]

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Stacked PR. Base is user/gudge/versioning_phase3a_decouple_backend (Phase 3a), not main. Review/merge the stack in order (2.5 -> 3a -> 3b); this branch will be retargeted to main as its base lands.

Summary

Phase 3b of the versioning remediation. It raises the config schema floor from 0.4.0-alpha to 0.6.0-alpha: the Rust parser and the SDK now reject configs declaring 0.4.0-alpha or 0.5.0-alpha as "older than supported" (the deferred Phase 1 p1b back-dating gate), and accept 0.6.0-alpha through the dev ceiling 0.8.0-alpha. This is safe because Phase 3a decoupled Windows backend selection from the schema version, so no runtime behavior depends on the 0.4-vs-0.5 boundary anymore.

The released 0.4.0-alpha / 0.5.0-alpha stable schema files are kept as immutable historical artifacts (the repo convention never edits released stable schemas, and external configs may still reference them via $schema for editor validation); the parser simply stops accepting those versions.

Details

• Canonical source schemas/schema-version.json: min 0.4.0-alpha -> 0.6.0-alpha.
• Rust config_parser.rs: SUPPORTED_VERSION ">=0.4, <=0.8" -> ">=0.6, <=0.8", and the second hardcoded older-vs-newer floor classifier ">=0.4" -> ">=0.6". Converted the 0.4/0.5 acceptance tests into rejection tests (asserting older than supported); bumped incidental version literals.
• SDK sandbox.ts: MIN_VERSION -> 0.6.0-alpha. Flipped the unit-test bound case to assert 0.5.0-alpha is rejected; bumped incidental literals; bumped the integration-test version matrix (test-helpers.ts) and the hardcoded versions in the seatbelt/microvm/wslc integration tests.
• Corpus: migrated 80 configs in tests/configs + tests/examples from 0.4.0-alpha/0.5.0-alpha -> 0.6.0-alpha; renamed hello_world_v050.json -> hello_world_v060.json (containerId + message updated).
• Manual smoke scripts (T3-Workloads, WinProcessContainer-Tests, run_isolation_session_resize_smoke) bumped to 0.6.0-alpha.
• Docs: sdk/README.md compatibility table (0.4/0.5 marked retired below the floor) + examples + troubleshooting; docs/schema.md version-range table; docs/versioning.md, docs/playground-limitations.md, .github/copilot-instructions.md, assorted example snippets; check-schema-versions.js stale comment.

Deferred (follow-up, not in this PR)

• tests/playground still encodes the obsolete version-as-AppContainer/BaseContainer switch (default 0.5.0-dev). It needs a UI rework rather than a string bump and is not CI-gated; tracked as a follow-up.

Tests

• check-schema-versions.js, check-schema-codegen.js, check-sdk-types-codegen.js, validate-configs.js (169/169) all green.
• cargo test -p wxc_common (357 pass; 0.4/0.5 now rejected, 0.6+ accepted); cargo clippy --workspace --all-targets -- -D warnings; cargo fmt --all -- --check.
• cd sdk && npm run build + test:unit (181 pass); integration tests type-check.

Microsoft Reviewers: Open in CodeFlow