fix(adapter-slack): replace empty table cells with single space

[goofansu]

Summary

Slack's Block Kit API rejects table cells with empty text fields (must be more than 0 characters). When a markdown table contains an empty cell — e.g. a trailing | and more... | | row — the adapter passes text: "" to the API, causing an invalid_blocks error and the message failing to send.

Fixed in two places:

• packages/adapter-slack/src/markdown.ts — mdastTableToSlackBlock: ...join("") || " "
• packages/adapter-slack/src/cards.ts — convertTableToBlocks: convertEmoji(header) || " " and convertEmoji(cell) || " "

Empty cells now fall back to a single space, which satisfies the API constraint.

Test plan

Added a failing test to each affected file that reproduces the exact error, then verified the fix makes them pass:

• markdown.test.ts — toBlocksWithTable with | and more... | | (the cell that triggered the original crash at blocks/1/rows/15/1). Asserts every cell has length > 0 and the empty cell becomes " ".
• cards.test.ts — convertTableToBlocks with an empty header cell and an empty data cell. Same assertions.

Both tests were confirmed red without the fix and green with it.

Original error:

[ERROR]  web-api:WebClient:0 failed to match any allowed schemas [json-pointer:/blocks/1/rows/15/1]
[ERROR]  web-api:WebClient:0 must be a valid enum value [json-pointer:/blocks/1/rows/15/1/type]
[ERROR]  web-api:WebClient:0 must be more than 0 characters [json-pointer:/blocks/1/rows/15/1/text]
[ERROR]  web-api:WebClient:0 missing required field: elements [json-pointer:/blocks/1/rows/15/1]
[chat-sdk] Message processing error {
  error: Error: An API error occurred: invalid_blocks
      at platformErrorFromResult (/Users/james/code/pi-chat/node_modules/.pnpm/@slack+web-api@7.15.0/node_modules/@slack/web-api/dist/errors.js:67:33)
      at WebClient.apiCall (/Users/james/code/pi-chat/node_modules/.pnpm/@slack+web-api@7.15.0/node_modules/@slack/web-api/dist/WebClient.js:232:56)
      at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
      at async _SlackAdapter.editMessage (file:///Users/james/code/pi-chat/node_modules/.pnpm/@chat-adapter+slack@4.23.0/node_modules/@chat-adapter/slack/dist/index.js:2571:25)
      at async Object.edit (file:///Users/james/code/pi-chat/node_modules/.pnpm/chat@4.23.0/node_modules/chat/dist/index.js:1669:9)
      at async askPi (file:///Users/james/code/pi-chat/index.ts:258:2)
      at async file:///Users/james/code/pi-chat/index.ts:267:2
      at async Chat.runHandlers (file:///Users/james/code/pi-chat/node_modules/.pnpm/chat@4.23.0/node_modules/chat/dist/index.js:3294:7)
      at async Chat.dispatchToHandlers (file:///Users/james/code/pi-chat/node_modules/.pnpm/chat@4.23.0/node_modules/chat/dist/index.js:3156:7)
      at async Chat.handleQueueOrDebounce (file:///Users/james/code/pi-chat/node_modules/.pnpm/chat@4.23.0/node_modules/chat/dist/index.js:3018:9) {
    code: 'slack_webapi_platform_error',
    data: {
      '0': 'failed to match any allowed schemas [json-pointer:/blocks/1/rows/15/1]',
      '1': 'must be a valid enum value [json-pointer:/blocks/1/rows/15/1/type]',
      '2': 'must be more than 0 characters [json-pointer:/blocks/1/rows/15/1/text]',
      '3': 'missing required field: elements [json-pointer:/blocks/1/rows/15/1]',
      ok: false,
      error: 'invalid_blocks',
      response_metadata: [Object]
    }
  },
  threadId: 'slack:C0AQLMSJVQD:1775264513.393679'
}

Triggered by a table with a trailing empty cell:

| Kind            | Label               |
|-----------------|---------------------|
| FORM            | Form Submission     |
| ...             | ...                 |
| and more...     |                     |

Encounter the problem and fix in this commit: goofansu/pi-chat@1199a1a

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