feat: add update_block tool to edit existing block content by UUID

[ergut]

Summary

Add an update_block tool that edits the content of an existing block by UUID. This is the only missing piece of block-level CRUD — read (get_page_content), create (insert_nested_block), and delete (delete_block) are all covered.

Motivation

Currently there's no way to modify an existing block's text without deleting and recreating it. Common use cases:

• Fix a typo in a specific block
• Update a task's status marker
• Edit a block that was found via search or query

Proposed API

Tool name: update_block

Input schema:

{
  "block_uuid": "string (required) — UUID of the block to edit",
  "content": "string (required) — new content to replace the block's current text"
}

Underlying Logseq API: logseq.Editor.updateBlock(blockUUID, content)

Implementation Notes

• logseq.Editor.updateBlock is part of the plugin API and should be available via HTTP
• Returns null on success (like removeBlock)
• Should follow the existing emoji-prefix pattern: ✅ on success, ❌ on error
• Needs unit tests in test_logseq_api.py and test_tool_handlers.py

Acceptance Criteria

• LogSeq.update_block(block_uuid, content) method in logseq.py
• UpdateBlockToolHandler in tools.py
• Registered in server.py
• Unit tests for both the API method and the handler
• Integration test expected_tools updated (15 tools)
• README tool table updated

[ergut]

Shipped in v1.4.0.