markdown block type not supported when sending via WebhookClient / response_url

从 slack_sdk.webhook.WebhookClient.send() 及其同步/异步实现开始，然后将它们的 block 处理与 WebClient.chat_postMessage() 进行比较。针对两个 endpoint 运行提供的 markdown-block 示例，并检查响应行为。当 WebhookClient 一致地支持 markdown block 类型，或明确识别出 endpoint 的限制并通过测试覆盖该限制时，即视为完成。

The markdown block type (introduced in Block Kit - Feb 2025, expanded in Mar 2026) works correctly when sent via WebClient.chat_postMessage, but returns a 500 error when sent via WebhookClient (i.e. slash command
response_url).

Reproducible example:

from slack_sdk.webhook import WebhookClient
from slack_sdk import WebClient

blocks = [
    {
        "type": "markdown",
        "text": "## Heading\n\n| Col A | Col B |\n|---|---|\n| 1 | 2 |"
    }
]

# ✅ Works — returns 200
client = WebClient(token="xoxb-...")
client.chat_postMessage(channel="#test", blocks=blocks, text="fallback")

# ❌ Fails — returns 500
webhook = WebhookClient("https://hooks.slack.com/commands/T.../...")
webhook.send(blocks=blocks)

Expected behavior: WebhookClient.send() should support the markdown block type, same as chat.postMessage.

Actual behavior: Slack's response_url endpoint returns HTTP 500 with no descriptive error body.

Context:

• slack_sdk version: 3.39.0
• Python version: 3.12.12
• This affects slash command responses where response_url is the standard reply mechanism

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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