docs(protocol): name the 429 backoff member what the runtime actually emits (#16938)

`content/docs/protocol/kernel/error-handling.mdx` taught `details.retry_after`
on three error envelopes and instructed clients to read
`data.error.details.retry_after`. No producer emits that member: both 429
emitters build `details: { retryAfterSeconds, resetAt }`
(`packages/runtime/src/endpoint-policy.ts`,
`packages/runtime/src/security/inbound-rate-limit.ts`), and two tests pin the
spelling. A client written from this page reads `undefined` on every real 429
and silently falls back to its own backoff against a service that just asked
it to wait.

Only `details`-bag members move. The `RetryStrategy` enum value `retry_after`
is a different, correct use of the same token and is untouched in all seven of
its sites (`content/docs/api/error-catalog.mdx` x4,
`content/docs/references/api/errors.mdx` x3).

Claude-Session: https://claude.ai/code/session_016N6xmWt5hYm94ffVEwGH8x

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]

content/docs/protocol/kernel/error-handling.mdx

@@ -420,7 +420,7 @@ never crosses HTTP. The refusal is emitted as the flat body shown above, and
     "details": {
       "limit": 1000,
       "window": "1m",
-      "retry_after": 45,
+      "retryAfterSeconds": 45,
       "quota_reset": "2024-01-16T14:31:00Z"
     }
   }
@@ -528,7 +528,7 @@ async function fetchWithRetry(url, options = {}, maxRetries = 3) {
     "message": "Service temporarily unavailable",
     "details": {
       "reason": "database_maintenance",
-      "retry_after": 300,
+      "retryAfterSeconds": 300,
       "estimated_completion": "2024-01-16T15:00:00Z"
     }
   }
@@ -652,7 +652,7 @@ Content-Type: application/json
     "details": {
       "limit": 1000,
       "window": "1m",
-      "retry_after": 45,
+      "retryAfterSeconds": 45,
       "quota_reset": "2024-01-16T14:31:00Z",
       "upgrade_url": "https://app.acme.com/billing/upgrade"
     },
@@ -736,7 +736,7 @@ async function fetchWithRetry(url, options = {}, maxRetries = 3) {
       if (!response.ok) {
         // Check if error is retryable
         if (data.error.code === 'RATE_LIMIT_EXCEEDED') {
-          const retryAfter = data.error.details.retry_after || 1;
+          const retryAfter = data.error.details.retryAfterSeconds || 1;
           await sleep(retryAfter * 1000);
           continue;
         } else if (data.error.code === 'INTERNAL_ERROR') {