feat(ossie/ai): synonym resolution + custom_extensions passthrough (#1408, #1409)

[buggtb]

Wires the OSI ai_context.synonyms and custom_extensions[] blocks
through the DTO layer, the AI schema view, and the query validator so
agents can refer to entities by synonym and consumer-side tooling can
carry vendor annotations without dropping them.

saiku#1408 — synonyms

• OssieModelDto gains fieldAliases / metricAliases / datasetAliases
  maps. OssieDiscoverService populates them via the library's
  bi.saiku.ossie.OssieSynonymIndex — dogfooding the ossie-core helper
  we shipped in feat: OssieSynonymIndex + strict-version-check ossie#4.
• OssieAiSchemaProjector copies the alias maps to the AI schema view
  so they surface on GET /ai/ossie/schema.
• OssieAiValidator resolves synonyms before rejecting names on
  POST /ai/ossie/query:
  • Dataset synonyms rewrite ref.dataset in place; downstream code sees
    the canonical name only.
  • Metric synonyms rewrite ref.metric similarly.
  • Field synonyms only rewrite when the alias resolves to a field on
    the SAME dataset the caller asked for — cross-dataset synonyms
    (ambiguous by design) get rejected instead of silently misrouting.

saiku#1409 — custom_extensions

• New CustomExtensionDto in org.saiku.service.ossie — thin wire model
  carrying vendor_name + parsed JSON data.
• OssieModelDto.Field / Dataset / Metric gain customExtensions lists;
  discover service parses each extension's opaque data string into a
  JsonNode once so consumers walk it structurally.
• 'visibility: internal' filter: any extension whose parsed data
  declares visibility=internal is dropped at the discover-service
  boundary. That's the standard escape hatch for tooling-private
  metadata (Saiku's own annotations, dbt's tool-only bookkeeping)
  that shouldn't reach an agent-facing view.
• OssieAiSchema.Field / Dataset / Metric gain customExtensions
  passthrough; projector copies straight across.

Verification

• Full reactor tests: BUILD SUCCESS
• saiku-service: 769/769 (was 760; +9 new tests)
  • OssieAiValidatorTest: +4 synonym-resolution cases (metric alias
    rewrite, dataset alias rewrite, field alias rewrite when dataset
    matches, field alias IGNORED when cross-dataset)
  • OssieAiSchemaProjectorTest: +3 (alias maps carry through, custom
    extensions carry through on field, on dataset + metric)
  • OssieDiscoverServiceTest: +2 (synonym aliases populated from
    ai_context, visibility:internal extensions filtered)
• saiku-sql: SaikuOssieConnectionTest still 2/2
• Zero behavioural regressions on existing tests

Docs

docs/AI-OSSIE-API.md gains two new subsections under Step 2:

• "AI-context synonyms (ossie/spec: parse + surface ai_context.synonyms #1408)" — shows fieldAliases/metricAliases/
  datasetAliases shape + describes the request-rewriting behaviour +
  documents the cross-dataset ambiguity guard
• "Custom extensions passthrough (ossie/spec: pass through custom_extensions in the AI schema view #1409)" — shows the customExtensions
  shape + the visibility:internal filter with a YAML example