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>

tools/test-integration.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+set -euo pipefail
+
+# Run a compiled *.test binary. When TEST_SHARD is set, enumerate top-level
+# tests via -test.list and run only the shard's slice; TestMain skips
+# environment setup in -test.list mode. Without TEST_SHARD, runs the binary
+# directly.
+
+BINARY=${1:?usage: $0 BINARY}
+
+if [ -z "${TEST_SHARD:-}" ]; then
+  exec "$BINARY"
+fi
+
+if ! [[ "${TEST_TOTAL_SHARDS:-}" =~ ^[1-9][0-9]*$ ]]; then
+  echo "TEST_TOTAL_SHARDS must be a positive integer, got: ${TEST_TOTAL_SHARDS:-}" >&2
+  exit 2
+fi
+if ! [[ "$TEST_SHARD" =~ ^[1-9][0-9]*$ ]] || [ "$TEST_SHARD" -gt "$TEST_TOTAL_SHARDS" ]; then
+  echo "TEST_SHARD must be in [1, $TEST_TOTAL_SHARDS], got: $TEST_SHARD" >&2
+  exit 2
+fi
+
+NAMES=$("$BINARY" -test.list='^Test' | LC_ALL=C sort -u | awk -v r=$((TEST_SHARD - 1)) -v t="$TEST_TOTAL_SHARDS" '(NR - 1) % t == r')
+if [ -z "$NAMES" ]; then
+  echo "shard $TEST_SHARD/$TEST_TOTAL_SHARDS has no tests assigned" >&2
+  exit 1
+fi
+PATTERN=$(echo "$NAMES" | paste -sd '|' -)
+echo "Running shard $TEST_SHARD/$TEST_TOTAL_SHARDS ($(echo "$NAMES" | wc -l | tr -d ' ') tests)"
+exec "$BINARY" -test.run "^($PATTERN)\$"