fix(gateway): close billing, limit, routing and translation gaps

Billing:
- Passthrough SSE streams now bill the usage the upstream reports.
  The gateway falls back to a request-size estimate only on a 2xx
  stream or a truncated JSON body.
- A unified stream that fails after it starts is estimated, not
  billed at zero.
- Billing uses the built-in LiteLLM price table and honours
  modelMeta free: true.
- Pricing entries are validated. Cost arithmetic rounds the total and
  saturates on overflow.
- Gemini thinking tokens count as completion tokens.

Limits and storage:
- A rejected request rolls back its counts on the other scopes, so one
  user cannot lock out a group.
- Redis EXPIRE replies are checked. Keys recreated without a TTL are
  repaired at once.
- A mutating pipeline is not retried after reply bytes arrive.
- History, model totals and target usage report a store outage.

Routing and translation:
- Discovered ids with a provider-like prefix are listed only in
  owner/id form, so listing and routing agree.
- Anthropic and Gemini discovery follows pagination.
- The developer role maps to system.
- stream_options.include_usage is honoured on translated streams.
- tool_choice none keeps tools when history holds tool blocks.
- refusal maps to content_filter. Data URI parameters are stripped.

Streaming, retry and failover:
- SSE accepts CR, LF and CRLF line ends. This closes a CR injection
  path.
- Retry-After is never jittered. A cancelled wait returns the
  context error.
- The watchdog timer stops at end of body.
- Oversized bodies return 413.
- A failed stream ends with an error event.
- Failover restores pre-existing headers. Client cancels do not count
  as provider successes.

Also: "admin" is a reserved provider name. metrics.path must start
with "/". Pricing warnings go to the right gateway instance. The admin
targets view sends access as null or [] so the two states differ.
Upstream organisation headers are stripped. The users-file reload
error is logged once per error. Dead code is removed, and test-only
helpers move to test files.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

testdata/gemini/chat_developer_role_in.json

@@ -0,0 +1,7 @@
+{
+  "model": "gemini-2.5-pro",
+  "messages": [
+    {"role": "developer", "content": "Always answer in one sentence."},
+    {"role": "user", "content": "Hello there"}
+  ]
+}

testdata/gemini/chat_developer_role_want.json

@@ -0,0 +1,6 @@
+{
+  "systemInstruction": {"parts": [{"text": "Always answer in one sentence."}]},
+  "contents": [
+    {"role": "user", "parts": [{"text": "Hello there"}]}
+  ]
+}

testdata/gemini/resp_thoughts_in.json

@@ -0,0 +1,19 @@
+{
+  "candidates": [
+    {
+      "content": {
+        "role": "model",
+        "parts": [{"text": "The answer is 42."}]
+      },
+      "finishReason": "STOP"
+    }
+  ],
+  "usageMetadata": {
+    "promptTokenCount": 10,
+    "candidatesTokenCount": 6,
+    "thoughtsTokenCount": 500,
+    "totalTokenCount": 516
+  },
+  "modelVersion": "gemini-2.5-pro-001",
+  "responseId": "resp_THINK1"
+}

testdata/gemini/resp_thoughts_want.json

@@ -0,0 +1,14 @@
+{
+  "id": "chatcmpl-resp_THINK1",
+  "object": "chat.completion",
+  "created": 1734000000,
+  "model": "gw-gemini-model",
+  "choices": [
+    {
+      "index": 0,
+      "message": {"role": "assistant", "content": "The answer is 42."},
+      "finish_reason": "stop"
+    }
+  ],
+  "usage": {"prompt_tokens": 10, "completion_tokens": 506, "total_tokens": 516}
+}