chore(java): use IR discriminatorContext for SSE event-level discrimination

[tstanmay13]

Description

Refs #12242

Refactors the Java SDK generator's SSE event-level discrimination logic to use the IR's discriminatorContext field (added in PR #12242) instead of the manual heuristic that checked if a discriminator property name matched SSE envelope fields (event, id, retry, data).

Link to Devin run: https://app.devin.ai/sessions/76958a26d45348e6ab03ffbbc7647ad2
Requested by: @tstanmay13

Changes Made

• AbstractGeneratorCli: Extracts discriminatorContext from raw IR JSON before Jackson deserialization (the irV63 Java model doesn't expose this field, but it's present in the JSON). New IrLoadResult wrapper, loadIr() replaces getIr(), and extractDiscriminatorContexts() walks the JSON tree for union types.
• AbstractGeneratorContext / ClientGeneratorContext: New constructor overload and getter to thread Map<TypeId, String> discriminatorContexts through the context hierarchy.
• Cli: All 3 ClientGeneratorContext construction sites pass getDiscriminatorContexts().
• SseDiscriminationAnalyzer: Core change — replaces SSE_ENVELOPE_FIELDS.contains(...) with a lookup into the discriminator contexts map. "protocol" → event-level, "data" or absent → data-level. resolveUnionType now returns UnionResolutionResult (TypeId + union) instead of just the union declaration.
• AbstractHttpResponseParserGenerator: Updated caller to pass discriminator contexts.
• versions.yml: New 3.36.2 entry.

Testing

• Java compilation passes (./gradlew compileJava)
• Spotless formatting passes (./gradlew spotlessCheck)
• No new unit tests added — relies on existing seed test coverage

Human Review Checklist

• Raw JSON extraction approach: extractDiscriminatorContexts manually walks the IR JSON tree. This is necessary because the Java IR model (v63) doesn't expose discriminatorContext. Verify the JSON path (types -> {id} -> shape -> discriminatorContext) matches the actual IR structure.
• Default fallback behavior change: Previously any discriminator named event/id/retry/data was treated as event-level. Now only types explicitly marked discriminatorContext: "protocol" in the IR get event-level treatment. If discriminatorContext is absent, it defaults to "data". Confirm this is the intended semantic for older IRs that predate the field.
• Runtime SseEventParser.java unchanged: The resource template still has its own SSE_ENVELOPE_FIELDS for runtime parsing — this is intentional (separate concern from build-time analysis).
• No test coverage: The new extraction logic and refactored analyzer have no unit tests. Seed tests should catch regressions, but manual verification may be needed.

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