ci: shard tests and reduce redundant work (#37618)

Critical path ~25:42 → ~19:56 (−22%), ~0% CI minutes.

- `test-pgsql` shards 2-way. Branch protection: replace `test-pgsql`
with `test-pgsql-shards (1)` + `test-pgsql-shards (2)`; `test-unit`,
sqlite/mysql/mssql unchanged — pgsql dominates the critical path.
- `test-unit` runs `bindata` then `bindata gogit` sequentially.
cache-seeder pre-warms the race-instrumented test compile cache and the
integration test binary so PR jobs warm-start.
- Cache writes restricted to cache-seeder; PR jobs use
`actions/cache/restore`. Defends against PR cache poisoning and frees
the 10 GB cap from PR churn.
- `go-cache` action: dropped the `cache-name` input. One gobuild cache,
one golangci-lint cache. Seeder lint job restores but doesn't save
gobuild, so only one writer populates it.
- `tools/test-integration.sh` shards the integration binary via
`-test.list`; `TestMain` short-circuits DB init in list mode.

`TestAPILFSNotStarted` / `TestAPILFSLocksNotStarted` switched to
`test.MockVariableValue` — latent `setting.LFS.StartServer` global-state
leak uncovered by sharding.

---
This PR was written with the help of Claude Opus 4.7

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>

.github/workflows/pull-compliance.yml

@@ -27,7 +27,6 @@ jobs:
           cache: false
       - uses: ./.github/actions/go-cache
         with:
-          cache-name: lint-backend
           lint-cache: "true"
       - run: make deps-backend deps-tools
       - run: TAGS="bindata" make generate-go # lint-go also lints with "bindata" tags which requires "_bindata.go"
@@ -75,9 +74,6 @@ jobs:
           check-latest: true
           cache: false
       - uses: ./.github/actions/go-cache
-        with:
-          cache-name: checks-backend
-          build-cache: "false"
       - run: make deps-backend deps-tools
       - run: make --always-make checks-backend # ensure the "go-licenses" make target runs
 
@@ -111,8 +107,6 @@ jobs:
           check-latest: true
           cache: false
       - uses: ./.github/actions/go-cache
-        with:
-          cache-name: compliance-backend
       - run: make deps-backend generate-go
       # no frontend build here as backend should be able to build, even without any frontend files
       # CGO is not used when cross-compile, so these steps also test if the code is compatible with CGO disabled