refactor(resolver): relocate httpresolver test double out of product surface

[snkmcb]

Summary

The repository-local HTTP resolver lived under plugins/ despite being an integration-test double. That placement put it in the product surface, the release metadata matrix, and its own standalone CI cells.

This moves it to tests/plugins/httpresolver and removes it from the product build:

• Dropped the top-level add_subdirectory(plugins/httpresolver).
• Removed its assertions from tools/check_release_metadata.py.
• Removed its standalone CI cells from openstrata.ci.yaml and ost-source-ci.yml.
• Deleted openstrata.plugin.yaml — it is no longer a published plugin.
• Dropped openstrata_install_plugin_bundle, which fixes a real prior bug: the double was being installed into the shipped COPC product stage.
• The COPC Tier 1 integration test builds it transitively under USDGEO_BUILD_TESTS in the root build, so the local gate stays independent of external resolver repositories.

Also included:

• Added TryBuildPointCloudCacheLayout so producer and consumer tests derive resolver-backed cache entries from the same descriptor contract.
• Reworked resolver cache Tier 1 coverage to assert cache hits, incomplete and corrupted entry invalidation, and validation-token changes through cache artifacts rather than process-local counters. The counters are not shared across the FileFormat DLL boundary on Windows, so they could not observe the behavior under test.
• Docs updated for the released usd-http-resolver implementation and its resolver-neutral ArAssetInfo identity contract.

Known gap: Tier 1 is not in the CI matrix

Review of the first commit caught that it claimed Tier 1 was the "required CI gate" and that the fixture was built transitively by CI. Neither is true, and the second commit corrects the docs rather than overstating coverage.

Every cell in openstrata.ci.yaml is a per-plugin bundle build rooted at plugins/pointcloud-<x>/CMakeLists.txt, which is a standalone project() that never declares the USDGEO_BUILD_TESTS option. Undefined there means the guarded block never runs, so neither pointcloudCopc_tests nor the resolver fixture is compiled in CI. The option defaults ON only in the root CMakeLists.txt, which is what the local ost configure && ost build && ost test gate configures.

Two consequences worth naming explicitly:

• The test-run gap predates this PR — pointcloudCopc_tests was always behind the guard, so Tier 1 has never run in CI.
• This PR does narrow compile coverage: the previous unconditional add_subdirectory meant COPC cells at least compiled HttpResolver.cpp. Now nothing in CI does.

Wiring Tier 1 into CI needs a new root-build cell with ctest, which costs billed runner minutes and is left as follow-up.

Test plan

• python tools/check_release_metadata.py → consistent at 0.9.0
• Verified no doc still claims Tier 1 is a CI gate
• Local ost configure && ost build && ost test to exercise Tier 1
• CI: confirm no cell still references plugins/httpresolver

Note: I did not run a local C++ build; the CMake path changes are unverified by CI for the reason above.

Follow-ups

• Wire Tier 1 into the CI matrix (root build + ctest).
• tests/test_pointcloud_copc.cpp writes cache.manifest as the literal committed\n, while the real producer (WriteManifest, tools/usd-pointcloud-convert/main.cpp:442) writes a structured manifest with payload enumeration. It passes only because InspectMarker checks existence and file type, never content — so Tier 1 does not exercise the producer/consumer manifest contract.

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