Backport read-only mode to 0.2.x

[eazyhozy]

Summary

Backport read-only mode from `main` to the `0.2.x` release branch, so it can be included in a v0.2.1 patch release.

Changes

• Cherry-pick d936f56 — Add read-only mode to reject write operations (Add read-only mode to reject write operations #230)
• Cherry-pick f0a84e8 — Skip write warmup in read-only mode (Skip write warmup in read-only mode #239)
• Add `POST /graph/v2/query` to `READ_ENDPOINTS` in `ReadOnlyRequestFilterTest` (0.2.x-specific: this endpoint was removed on `main` via Move query endpoint to v3 and remove unused v2 endpoint #228 but still exists on 0.2.x; the filter's `readSuffixes` already includes `/query` so no filter logic change is needed)

Note: PR #236 (Fix ReadOnlyRequestFilterTest for renamed edges/cache → edges/seek endpoint) is intentionally not included — the `edges/cache → edges/seek` rename has not been applied to 0.2.x, so applying #236 would break the test suite here.

How to Test

./gradlew :server:test --tests "*ReadOnly*" --tests "*StartUp*"
./gradlew build

[eazyhozy]

@em3s

CI was not triggered because the base branch is 0.2.x instead of main. Verified locally: all 269 server tests pass with no failures.

./gradlew :server:test --rerun-tasks
→ 269 tests completed, 0 failures, 0 errors — BUILD SUCCESSFUL