feat(ci): xdist per-worker isolation infrastructure

[mchen-sentry]

When pytest-xdist spawns multiple workers inside a single CI shard, all workers share ci infrasturcture (Redis, Kafka, Snuba, and Relay). Without isolation, workers corrupt each other (e.g. reset_snuba/flushdb(), Kafka events pollute, identical snowflake IDs).

This PR gives each worker its own isolated resources. Everything is gated on xdist env vars, so this change will be a no-op on master as is.

Changes

• xdist.py - resolves worker ID at module level, provides helpers (e.g. get_redis_db())
• per worker Redis (each worker gets its own Redis DB), per worker Kafka topics (topic names suffixed with worker ID), per worker Snuba (per worker ports), per worker Relay (per worker ports, unique contianers)
• deterministic region name so all workers generate the same region name (reequired for xdist identical test collection)
• per worker snowflake IDs to avoid collisions
• disable crash recovery in pytest-rerunfailures as this is broken under xdist due to Sentry's global socket.setdefaulttimeout(5), normal --reruns unaffected

[linear-code]

DI-1712 xdist infra

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Falsy-zero check masks xdist worker 0 identity
  • Changed 'xdist._worker_num or 0' to 'xdist._worker_num if xdist._worker_num is not None else 0' in relay.py to properly distinguish worker 0 from non-xdist case, matching the pattern used throughout the codebase.

Or push these changes by commenting:

@cursor push 99ad560e14

Preview (99ad560e14)

diff --git a/src/sentry/testutils/pytest/relay.py b/src/sentry/testutils/pytest/relay.py
--- a/src/sentry/testutils/pytest/relay.py
+++ b/src/sentry/testutils/pytest/relay.py
@@ -68,7 +68,7 @@
     template_path = _get_template_dir()
     sources = ["config.yml", "credentials.json"]
 
-    worker_num = xdist._worker_num or 0
+    worker_num = xdist._worker_num if xdist._worker_num is not None else 0
     relay_port = ephemeral_port_reserve.reserve(ip="127.0.0.1", port=33331 + worker_num * 100)
 
     redis_db = xdist.get_redis_db()

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