Title: Intermittent "23 validation errors: Field required" on tool calls with missing function field

Description:
Tool calls intermittently fail with a Zod validation error indicating a missing required function field, even when the tool call appears to be properly formatted. This occurs sporadically during normal agent operations.

Error Message:

23 validation errors:
  {'type': 'missing', 'loc': ('body', 'tools', 0, 'function'), 'msg': 'Field required', 'input': {'name': 'read', 'description': 'Read the contents of a file...', 'input_schema': {'type': 'object', ...}}}

Frequency:

• ~20-30% of tool calls under load
• Higher frequency with rapid successive tool calls
• Occurs with multiple tool types (read, edit, exec)

Environment:

• OpenClaw Version: 2026.2.21-2
• Model: kimi-coding/k2p5
• OS: Linux 6.6.87.2-microsoft-standard-WSL2
• Node: v22.22.0
• Shell: bash

Steps to Reproduce:

1. Start an agent session with tool access
2. Execute multiple tool calls in rapid succession (e.g., read → edit → exec)
3. Observe intermittent validation failures

Expected Behavior:
Tool calls should consistently validate and execute without schema errors.

Actual Behavior:
Intermittent validation failures with "Field required" error for the function field, despite the tool appearing to be properly structured.

Workaround:
Retrying the failed tool call usually succeeds on the second attempt.

Additional Context:

• Error appears to be a Zod schema validation failure
• Missing function field suggests the tool call structure is being malformed somewhere in the pipeline
• Not related to tool-specific logic (occurs across different tool types)
• May be a race condition or serialization issue in the gateway/agent communication layer

Related Issues:

• Different from openai-codex provider: pipe-delimited tool call IDs written to transcript break Codex API validation #35528 (pipe-delimited tool call IDs)
• Different from Fix: prevent Invalid diff now finding less tool calls error #35379 (streaming tool call count regression)
• This appears to be a NEW schema validation issue

Labels: agents, gateway, bug, validation