refactor(uts): share test infra via testFixtures, move objects unit suite into :liveobjects

[sacOO7]

Summary

Follow-up to the cross-SDK objects audit. Three related changes, bundled because the test move and the production conformance work touch the same suites:

1. Share the UTS test infra — moved from :uts's src/test into its src/testFixtures variant, so other modules can consume it via testFixtures(project(":uts")).
2. Relocate the objects UTS unit suite into the :liveobjects module's own test source set, so the internal-graph specs can reach internal members directly (and expand coverage).
3. Production spec-conformance — op-path returns ObjectUpdate, plus two root-object safeguards.

Based on fix/liveobjects-objects-audit-op-handling (not main).

Related spec

Mirrors the spec changes in ably/specification#512 into the ably-java derived tests and source:

• RTO27 — objects-data lifecycle on channel state transitions (DETACHED/FAILED clear pooled data without emitting; SUSPENDED retains); new white-box unit test.
• RTLC9g / RTLM7f — op-path returns the LiveObjectUpdate; the internal-counter/map UTS tests assert the returned update directly.
• RTO10c1b1 / RTLO4e10 — root object excluded from GC and non-tombstoneable.
• RTO18d — duplicate-listener handling kept, with the optional note allowing SDKs that de-duplicate to assert call_count == 1.
• Nested pooled-map coverage in the RTO27 assertions (CodeRabbit follow-up on the JS side, kept in sync).

Changes

1. Test infra → testFixtures

• 16 infra files (mock WebSocket/HTTP transport, FakeClock, SandboxApp, proxy control) move to uts/src/testFixtures/kotlin/io/ably/lib/uts/infra/ — pure moves, no content change.
• :uts gains the java-test-fixtures plugin; fixture deps scoped testFixturesApi/testFixturesImplementation.
• Acyclicity invariant (documented in uts/build.gradle.kts): :liveobjects test → :uts testFixtures → :java, with :uts test → :liveobjects kept testRuntimeOnly. testFixtures must never depend on :liveobjects.

2. Objects UTS unit suite → :liveobjects

• Suite moves :uts → liveobjects/src/test/.../uts/unit/ (package io.ably.lib.liveobjects.uts.unit).
• Coverage expands with new suites: InternalLiveCounter, InternalLiveMap, ObjectId, ObjectsPool, ParentReferences.
• runLiveObjectsUnitTests now filters both io.ably.lib.liveobjects.unit.* and io.ably.lib.liveobjects.uts.unit.* — CI coverage preserved, no integration tiers pulled in.
• Deviations recorded in liveobjects/.../uts/deviations.md (manual's format; S/T shape/typed vocabulary).

3. Production spec-conformance (liveobjects/src/main)

• Op-path returns ObjectUpdate instead of Boolean (RTLC9g/RTLM7f). The RTO9a2a4 on-ack serial gate now uses !update.noOp, matching the UTS model where result == true ⟺ !update.noop.
• Root object excluded from GC (RTO10c1b1).
• Root object rejects tombstone attempts (RTLO4e10).
• Both safeguards covered by new tests (LiveObjectTombstoneTest).

Verification

• :liveobjects:runLiveObjectsUnitTests + :uts:runUtsUnitTests — green, unit-only (0 integration tests).
• checkWithCodenarc checkstyleMain checkstyleTest — green.

Follow-up TODOs (draft — from FUTURE_WORK_UTS_INFRA.md)

Deferred intentionally; do not do speculatively. Trigger = a second infra consumer beyond :liveobjects.

• Extract shared infra into a dedicated :test-support module (rename pkg io.ably.lib.uts.infra → io.ably.lib.testsupport); drop java-test-fixtures from :uts.
• Add a Kotlin test source set to :java for its UTS-derived tests — guardrail: keep kotlin-stdlib out of :java's main artifact.
• Decide Chat SDK consumption: in-repo module vs. separate-repo publish/version.
• Simplify uts-package-mapping.json to one full path per tier (drop testRoot + {root,path} form; simplify resolve_uts.py).

Notes for reviewers

Read as three changes, not move noise — the op-path return and the root-object safeguards are behavioural. FUTURE_WORK_UTS_INFRA.md captures the deferred extraction and its rationale.

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