feat(#919): adopt session.usage WS push event for real-time context & compression updates (#927)

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/data/model/UsageSnapshotResponse.kt

@@ -1,30 +1,42 @@
 package com.m57.hermescontrol.data.model
 
 /**
- * Minimal view of the `session.usage` WS RPC result — only the fields the
- * context widget consumes today. The backend sends many more keys
+ * Minimal view of the `session.usage` WS RPC result or `session.usage` push event —
+ * only the fields the context widget consumes today. The backend sends many more keys
  * (`input`/`output`/`total`/`calls`/`active_subagents`/`credits_lines` …);
  * parsing is key-by-key, so unmodelled keys are simply ignored.
  *
  * Verified backend shape (`tui_gateway/server.py` `_get_usage`): `compressions`
  * = `context_compressor.compression_count` (0 when never compressed, absent on
- * sessions with no live agent).
+ * sessions with no live agent), `context_used` = current window tokens,
+ * `context_max` = max context length.
  */
 data class UsageSnapshotResponse(
     /** How many times this session has been context-compressed. */
-    val compressions: Int?,
+    val compressions: Int? = null,
+    /** Current prompt / context tokens used in the active window. */
+    val contextUsed: Long? = null,
+    /** Max context length in tokens. */
+    val contextMax: Long? = null,
+    /** Cumulative session total tokens. */
+    val totalTokens: Long? = null,
 )
 
 /**
- * Parse the decoded JSON-RPC result of `session.usage`.
+ * Parse the decoded JSON-RPC result of `session.usage` or `session.usage` push event payload.
  *
- * The WS event parser decodes JSON numbers as [Double] (`JsonRpcModels.toAny`),
- * so every count is read through [Number]. Returns null when the payload is
- * not a map (error/malformed response) — callers keep the last known value.
+ * Handles both direct usage dict (from RPC response) and nested `{ "usage": { ... } }`
+ * (from push event payload). The WS event parser decodes JSON numbers as [Double]
+ * (`JsonRpcModels.toAny`), so every count is read through [Number].
+ * Returns null when the payload is not a map (error/malformed response) — callers keep the last known value.
  */
 fun parseUsageSnapshot(result: Any?): UsageSnapshotResponse? {
     val map = result as? Map<*, *> ?: return null
+    val usageMap = (map["usage"] as? Map<*, *>) ?: map
     return UsageSnapshotResponse(
-        compressions = (map["compressions"] as? Number)?.toInt(),
+        compressions = (usageMap["compressions"] as? Number)?.toInt(),
+        contextUsed = (usageMap["context_used"] as? Number)?.toLong(),
+        contextMax = (usageMap["context_max"] as? Number)?.toLong(),
+        totalTokens = (usageMap["total"] as? Number)?.toLong(),
     )
 }