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/cache-seeder.yml

@@ -1,15 +1,9 @@
-# Populates the go module, build, and golangci-lint caches under the default
-# branch's cache scope so that PR runs have a warm fallback to restore from.
+# Populates main's cache scope so PR runs warm-start from it. Saves the go
+# module, go build (incl. test compile), and golangci-lint caches.
 #
-# GitHub Actions caches are scoped per ref: a PR run can only write to its own
-# branch's scope, but can read from the base branch's scope as a fallback.
-# PRs therefore cannot seed main's scope themselves. Running the same cache
-# steps on push-to-main is the only opportunity to populate that fallback
-# scope so fresh PR branches start with a useful cache on first run.
-
-# A PR job's exact key lives in its own PR-scope (empty on first run, filled
-# by later runs of the same PR); on miss, actions/cache's restore-keys fall
-# back to prefix matches against entries this seeder saves in main's scope.
+# Caches are ref-scoped: PR runs read their own scope then fall back to the
+# base branch. Per .github/actions/go-cache/action.yml, PRs are restore-only,
+# so push-to-main is the only opportunity to populate the fallback scope.
 
 name: cache-seeder
 
@@ -41,11 +35,25 @@ jobs:
           check-latest: true
           cache: false
       - uses: ./.github/actions/go-cache
-        with:
-          cache-name: seed
-      - run: make deps-backend
+      - run: make deps-backend deps-tools
       - run: TAGS="bindata" make backend
       - run: TAGS="bindata gogit" GOEXPERIMENT="" make backend
+      - name: warm test compile cache (bindata)
+        env:
+          TAGS: bindata
+          GOTEST_FLAGS: -race -list=^$$ -count=1
+        run: make test-backend
+      - name: warm test compile cache (bindata gogit)
+        env:
+          TAGS: bindata gogit
+          GOEXPERIMENT:
+          GOTEST_FLAGS: -race -list=^$$ -count=1
+        run: make test-backend
+      - name: warm integration compile cache
+        run: |
+          TAGS="bindata" make test-integration-compile
+          TAGS="bindata gogit" GOEXPERIMENT="" make test-integration-compile
+          TAGS="bindata gogit" GOTEST_FLAGS="-race" make test-integration-compile
 
   lint:
     runs-on: ubuntu-latest
@@ -53,7 +61,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { job: lint-backend, tags: "bindata", target: "lint-backend" }
+          - { tags: "bindata", target: "lint-backend" }
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
@@ -63,7 +71,6 @@ jobs:
           cache: false
       - uses: ./.github/actions/go-cache
         with:
-          cache-name: ${{ matrix.job }}
           lint-cache: "true"
       - run: make deps-backend deps-tools
       - run: make generate-go