fix(compression): include system prompt + tool schemas in token estimates (#18265)

The user-visible /compress banner and the post-compression last_prompt_tokens
writeback both counted only the raw message transcript (chars/4). With a 15KB
system prompt and 30 tool schemas (~26KB), a 4-message transcript that looks
like ~45 tokens to the transcript-only estimator is really ~10.5K tokens of
request pressure — a 234x gap.

Two user-facing consequences:
- Banner shows 'Compressing … (~45 tokens)…' while compression is actually
  firing on 10K+ tokens of real pressure, confusing users about why
  compression triggered (reported by @codecovenant on X; #6217).
- Post-compression last_prompt_tokens writeback omits tool schemas, so the
  next should_compress() check compares real usage against a stale
  underestimate — compression triggers late, potentially past the model's
  context limit on small-context models (#14695).

Swap estimate_messages_tokens_rough() for estimate_request_tokens_rough()
at every user-visible banner and at the post-compression writeback.
estimate_request_tokens_rough() already existed for exactly this purpose
and includes system prompt + tool schemas.

Touched call sites:
- run_agent.py: post-compression last_prompt_tokens writeback, post-tool
  call should_compress() fallback when provider usage is missing
- cli.py: /compress banner + summary
- gateway/run.py: gateway /compress banner + summary
- tui_gateway/server.py: TUI /compress status + summary
- acp_adapter/server.py: ACP /compact before/after

Left intentionally alone:
- Session-hygiene fallback and the 'no agent' /status path in gateway/run.py
  — no agent instance is in scope to query for system prompt/tools, and the
  existing 30-50% overestimate wobble on hygiene is safety-accepted.
- Verbose-mode 'Request size' logging — informational only, already counts
  system prompt via api_messages[0].

Also relabels the feedback line from 'Rough transcript estimate' to
'Approx request size' so the metric label matches what it actually measures.

Credits: diagnoses from @devilardis (#14695) and @Jackten (#6217);
user report @codecovenant on X (2026-04-30).

Closes #14695
Closes #6217

tests/cli/test_manual_compress.py

@@ -21,20 +21,21 @@ def test_manual_compress_reports_noop_without_success_banner(capsys):
     shell.agent = MagicMock()
     shell.agent.compression_enabled = True
     shell.agent._cached_system_prompt = ""
+    shell.agent.tools = None
     shell.agent.session_id = shell.session_id  # no-op compression: no split
     shell.agent._compress_context.return_value = (list(history), "")
 
-    def _estimate(messages):
+    def _estimate(messages, **_kwargs):
         assert messages == history
         return 100
 
-    with patch("agent.model_metadata.estimate_messages_tokens_rough", side_effect=_estimate):
+    with patch("agent.model_metadata.estimate_request_tokens_rough", side_effect=_estimate):
         shell._manual_compress()
 
     output = capsys.readouterr().out
     assert "No changes from compression" in output
     assert "✅ Compressed" not in output
-    assert "Rough transcript estimate: ~100 tokens (unchanged)" in output
+    assert "Approx request size: ~100 tokens (unchanged)" in output
 
 
 def test_manual_compress_explains_when_token_estimate_rises(capsys):
@@ -49,22 +50,23 @@ def test_manual_compress_explains_when_token_estimate_rises(capsys):
     shell.agent = MagicMock()
     shell.agent.compression_enabled = True
     shell.agent._cached_system_prompt = ""
+    shell.agent.tools = None
     shell.agent.session_id = shell.session_id  # no-op: no split
     shell.agent._compress_context.return_value = (compressed, "")
 
-    def _estimate(messages):
+    def _estimate(messages, **_kwargs):
         if messages == history:
             return 100
         if messages == compressed:
             return 120
         raise AssertionError(f"unexpected transcript: {messages!r}")
 
-    with patch("agent.model_metadata.estimate_messages_tokens_rough", side_effect=_estimate):
+    with patch("agent.model_metadata.estimate_request_tokens_rough", side_effect=_estimate):
         shell._manual_compress()
 
     output = capsys.readouterr().out
     assert "✅ Compressed: 4 → 3 messages" in output
-    assert "Rough transcript estimate: ~100 → ~120 tokens" in output
+    assert "Approx request size: ~100 → ~120 tokens" in output
     assert "denser summaries" in output
 
 
@@ -89,6 +91,7 @@ def test_manual_compress_syncs_session_id_after_split():
     shell.agent = MagicMock()
     shell.agent.compression_enabled = True
     shell.agent._cached_system_prompt = ""
+    shell.agent.tools = None
     # Simulate _compress_context mutating agent.session_id as a side effect.
     def _fake_compress(*args, **kwargs):
         shell.agent.session_id = new_child_id
@@ -97,7 +100,7 @@ def test_manual_compress_syncs_session_id_after_split():
     shell.agent.session_id = old_id  # starts in sync
     shell._pending_title = "stale title"
 
-    with patch("agent.model_metadata.estimate_messages_tokens_rough", return_value=100):
+    with patch("agent.model_metadata.estimate_request_tokens_rough", return_value=100):
         shell._manual_compress()
 
     # CLI session_id must now point at the continuation child, not the parent.
@@ -118,11 +121,12 @@ def test_manual_compress_no_sync_when_session_id_unchanged():
     shell.agent = MagicMock()
     shell.agent.compression_enabled = True
     shell.agent._cached_system_prompt = ""
+    shell.agent.tools = None
     shell.agent.session_id = shell.session_id
     shell.agent._compress_context.return_value = (list(history), "")
     shell._pending_title = "keep me"
 
-    with patch("agent.model_metadata.estimate_messages_tokens_rough", return_value=100):
+    with patch("agent.model_metadata.estimate_request_tokens_rough", return_value=100):
         shell._manual_compress()
 
     # No split → pending title untouched.