fix: count tool call arguments in the compaction tail budget

[liuzhengyang699]

Compaction counts only message content when deciding which history fits in the retained tail. Assistant tool calls commonly have empty content but large arguments, so an old file-writing call can be retained as if it consumed no context.

Include the serialized tool calls in the character estimate, as the runtime token estimator already does. The current-user-turn retention rule and tool-call pairing still apply.

Validation: 9 tests passed across tests/test_compaction_tool_call_budget.py and tests/test_session_compaction.py. The new old-call regression fails before the fix; a second case verifies that a large call in the active turn keeps its matching tool result. Ruff 0.15.21 check/format and git diff --check passed on the changed files.