10.0.0: make PolicyCategory an open value class, so an unknown category is typed rather than null

[saurabhjain1592]

What

PolicyTypes.PolicyCategory (src/main/java/com/getaxonflow/sdk/types/policies/PolicyTypes.java) is a closed enum with a @JsonValue getter only. The platform sends categories outside it: pdp/shipped_posture.json at enterprise 857455033 carries 14, five of which the enum lacks (security-dangerous, compliance-euaiact, dangerous_queries, pii_detection, sql_injection). A closed enum cannot represent a value this build does not know.

For 9.x the fix is additive, so nothing breaks on the release train:

• the enum gains the five constants;
• the read types keep the wire string, so a read-then-write round-trips byte-exact;
• getCategory() returns the enum, or null for a category this build does not know;
• getCategoryValue() is the lossless form;
• the builders gain a category(String) overload.

The null from getCategory() is a temporary cost of staying source-compatible, not the design.

The 10.0.0 shape

Make PolicyCategory an open value class, on the pattern authzen.AuthZENObligationType already uses:

• public static final constants for the known values;
• PolicyCategory.of(String), with @JsonCreator, returning the constant or a carrier for an unknown value;
• isKnown();
• @JsonValue value();
• equals and hashCode on the value.

An unknown category is then typed everywhere, in reads, requests and builders, instead of null, and the getCategoryValue() / category(String) pair from 9.x can be deprecated.

The change is source-breaking for callers that switch on the enum or use valueOf, values(), ordinal(), EnumSet or EnumMap. So it belongs in a major release. Inside the SDK the only enum-only uses today are four PolicyCategory.valueOf lines in MediaGovernanceTypesTest.

PolicyTier stays a closed enum: the platform constrains a static policy's tier to system, organization and tenant by a database CHECK (enterprise migrations/core/030_policy_tier_columns.sql:14 and :47).

Refs getaxonflow/axonflow-enterprise#4224 (the spec's category enum is staler than every SDK).