feat: store and detect Rego language version per rule type (Phase 1) #5262

[dakshhhhh16]

Problem

Minder's Rego evaluator unconditionally compiles all policies with ast.RegoV0, even on OPA v1.14+ where the recommended default is V1. This blocks users from writing modern Rego policies using if, import rego.v1, every, etc., and it means we have no foundation for a gradual V1 migration.

See parent issue: #5262 -- Rego V1 Support

Approach

Instead of a big-bang migration, this phase of the plan establishes the necessary infrastructure by:

1. Per-rule-type version storage -- adds a rego_version column to the rule_type table (defaulting to 'v0') so each rule type carries an explicit version marker.
2. Parse-attempt detection -- on every CreateRuleType / UpdateRuleType, the server attempts to parse the Rego definition with the V1 parser first. If it succeeds, rego_version = 'v1' is stored; otherwise 'v0' is stored. No user action required.
3. Dual-parse validation -- the protobuf Validate() path now tries V1 first and falls back to V0, so valid V1 or V0 policies both pass validation.
4. Feature-flag-gated evaluation -- when rego_v1_dual_parse is enabled, the Rego evaluator reads the stored version and passes it via rego.SetRegoVersion() rather than hardcoding V0.

Changes

Area	File(s)	What changed
DB migration	database/migrations/000117_rule_type_rego_version.{up,down}.sql	Adds rego_version TEXT NOT NULL DEFAULT 'v0' to rule_type
SQL / sqlc	database/query/rule_types.sql, internal/db/models.go, internal/db/rule_types.sql.go	rego_version wired through Create/Update queries; regenerated
Mock	database/mock/store.go	Regenerated via mockgen
Version helpers	internal/engine/eval/rego/version.go	DetectRegoVersion(), VersionToString(), VersionFromString()
Evaluator	internal/engine/eval/rego/eval.go	WithRegoVersion option + evaluator stores/uses version
Cache	internal/engine/rtengine/cache.go	Reads stored version behind rego_v1_dual_parse flag
Service	pkg/ruletypes/service.go	Calls DetectRegoVersion() on create/update
Validator	pkg/api/protobuf/go/minder/v1/validators.go	Dual-parse (try V1, fall back V0)
Feature flag	pkg/flags/constants.go	RegoV1DualParse experiment constant
Tests	internal/engine/eval/rego/version_test.go	12 unit tests for detection + roundtrip

Out of scope (future phases)

• Backfilling existing rule types via a migration job
• Per-rule-type rego_version surface in the API / CLI
• V0 deprecation warnings (rego_v1_warn_v0 flag)
• V0 refusal (rego_v1_refuse_v0 flag)