feat: add AI budget policy and period deployment config (#25122)

Closes
https://linear.app/codercom/issue/AIGOV-283/add-deployment-config-for-ai-budget-policy-and-period

Adds `CODER_AI_BUDGET_POLICY` and `CODER_AI_BUDGET_PERIOD` deployment
options for AI Governance cost controls.

cli/testdata/coder_server_--help.golden

@@ -173,6 +173,15 @@ AI BRIDGE OPTIONS:
           Emit structured logs for AI Bridge interception records. Use this for
           exporting these records to external SIEM or observability systems.
 
+      --ai-budget-period month, $CODER_AI_BUDGET_PERIOD (default: month)
+          Determines when accumulated AI spend resets to zero, aligned to UTC
+          calendar boundaries. Only "month" is currently supported.
+
+      --ai-budget-policy highest, $CODER_AI_BUDGET_POLICY (default: highest)
+          Determines the effective group when a user belongs to multiple groups
+          with AI budgets. "highest" selects the group with the largest spend
+          limit, and is currently the only supported value.
+
 AI BRIDGE PROXY OPTIONS: 
       --aibridge-proxy-dump-dir string, $CODER_AIBRIDGE_PROXY_DUMP_DIR
           Directory for dumping MITM request/response pairs to disk for

cli/testdata/server-config.yaml.golden

@@ -837,6 +837,15 @@ aibridge:
   # or re-open the circuit.
   # (default: 3, type: int)
   circuit_breaker_max_requests: 3
+  # Determines the effective group when a user belongs to multiple groups with AI
+  # budgets. "highest" selects the group with the largest spend limit, and is
+  # currently the only supported value.
+  # (default: highest, type: enum[highest])
+  budget_policy: highest
+  # Determines when accumulated AI spend resets to zero, aligned to UTC calendar
+  # boundaries. Only "month" is currently supported.
+  # (default: month, type: enum[month])
+  budget_period: month
 aibridgeproxy:
   # Enable the AI Bridge MITM Proxy for intercepting and decrypting AI provider
   # requests.