Add CLI minimum-version compatibility guard #1200

Problem

The Unity package and the native CLI intentionally use separate version lines:

• Packages/src/Cli~/contract.json declares the latest bundled CLI contract version.
• CliConstants.MINIMUM_REQUIRED_CLI_VERSION declares the oldest CLI version the Unity package accepts.

Because those values are intentionally allowed to differ, CI must not enforce minimum == latest.

However, compatibility-sensitive changes can still require raising MINIMUM_REQUIRED_CLI_VERSION. It is easy to forget that check when changing CLI behavior, Editor/CLI contracts, readiness flows, or installer/update behavior.

Proposal

Add a lightweight compatibility guard that makes the minimum CLI version decision explicit.

The guard should:

• Verify MINIMUM_REQUIRED_CLI_VERSION is not greater than the latest CLI contract version.
• Detect compatibility-sensitive changes where keeping the existing minimum version needs an explicit decision.
• Allow older minimum versions when they are intentional and documented.
• Avoid forcing every CLI release to become the new Unity package minimum.

One possible design is a small decision file, for example:

{
  "minimumRequiredCliVersion": "3.0.0-beta.9",
  "decision": "keep",
  "reason": "The changed CLI behavior is additive and older CLIs still satisfy the Editor contract."
}

If the package truly depends on newer CLI behavior, the fix should be to raise CliConstants.MINIMUM_REQUIRED_CLI_VERSION and update the focused tests instead of adding a keep decision.

Acceptance Criteria

• CI fails if MINIMUM_REQUIRED_CLI_VERSION is greater than the latest CLI contract version.
• CI does not fail just because MINIMUM_REQUIRED_CLI_VERSION is lower than the latest CLI contract version.
• CI fails for compatibility-sensitive changes when neither the minimum version nor an explicit keep decision is updated.
• The keep-decision path requires a short reason so the compatibility judgment is reviewable.
• The guard is covered by focused tests.

Context

PR #1199 needed a follow-up correction because the Unity package minimum CLI version still accepted older CLIs after the readiness flow stopped depending on server-state files.