fix(backend): Add channel ID support to SendDiscordMessageBlock for consistency with other Discord blocks (#11055)

## Problem

The `SendDiscordMessageBlock` only accepted channel names, while other
Discord blocks like `SendDiscordFileBlock` and `SendDiscordEmbedBlock`
accept both channel IDs and channel names. This inconsistency made it
difficult to use channel IDs with the message sending block, which is
often more reliable and direct than name-based lookup.

## Solution

Updated `SendDiscordMessageBlock` to accept both channel IDs and channel
names through the `channel_name` field, matching the implementation
pattern used in other Discord blocks.

### Changes Made

1. **Enhanced channel resolution logic** to try parsing the input as a
channel ID first, then fall back to name-based search:
   ```python
   # Try to parse as channel ID first
   try:
       channel_id = int(channel_name)
       channel = client.get_channel(channel_id)
   except ValueError:
       # Not an ID, treat as channel name
       # ... search guilds for matching channel name
   ```

2. **Updated field descriptions** to clarify the dual functionality:
- `channel_name`: Now describes that it accepts "Channel ID or channel
name"
   - `server_name`: Clarified as "only needed if using channel name"

3. **Added type checking** to ensure the resolved channel can send
messages before attempting to send

4. **Updated documentation** to reflect the new capability

## Backward Compatibility

✅ **Fully backward compatible**: The field name remains `channel_name`
(not renamed), and all existing workflows using channel names will
continue to work exactly as before.

✅ **New capability**: Users can now also provide channel IDs (e.g.,
`"123456789012345678"`) for more direct channel targeting.

## Testing

- All existing tests pass, including `SendDiscordMessageBlock` and all
other Discord block tests
- Implementation verified to match the pattern used in
`SendDiscordFileBlock` and `SendDiscordEmbedBlock`
- Code passes all linting, formatting, and type checking

Fixes https://github.com/Significant-Gravitas/AutoGPT/issues/10909

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<details>

<summary>Original prompt</summary>

> Issue Title: SendDiscordMessage needs to take a channel id as an
option under channelname the same as the other discord blocks
> Issue Description: with how we can process the other discord blocks we
should do the same here with the identifiers being allowed to be a
channel name or id. we can't rename the field though or that will break
backwards compatibility
> Fixes
https://linear.app/autogpt/issue/OPEN-2701/senddiscordmessage-needs-to-take-a-channel-id-as-an-option-under
> 
> 
> Comment by User :
> This thread is for an agent session with githubcopilotcodingagent.
> 
> Comment by User :
> This thread is for an agent session with githubcopilotcodingagent.
> 
> Comment by User 055a3053-5ab6-449a-bcfa-990768594185:
> the ones with boxes around them need confirmed for lables but yeah its
related but not dupe
> 
> Comment by User 264d7bf4-db2a-46fa-a880-7d67b58679e6:
> this might be a duplicate since there is a related ticket but not sure
> 
> Comment by User :
> This comment thread is synced to a corresponding [GitHub
issue](https://github.com/Significant-Gravitas/AutoGPT/issues/10909).
All replies are displayed in both locations.
> 
> 


</details>


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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* New Features
* Send Discord Message block now accepts a channel ID in addition to
channel name.
  * Server name is only required when using a channel name.
* Improved channel detection and validation with clearer errors if the
channel isn’t found.

* Documentation
* Updated block documentation to reflect support for channel ID or name
and clarify when server name is needed.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ntindle <8845353+ntindle@users.noreply.github.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Nicholas Tindle <ntindle@users.noreply.github.com>
Co-authored-by: Bently <Github@bentlybro.com>

docs/content/platform/blocks/discord.md

@@ -43,7 +43,7 @@ The block uses a Discord bot to log into a server, locate the specified channel,
 |-------|-------------|
 | Discord Bot Token | A secret token used to authenticate the bot with Discord |
 | Message Content | The text content of the message to be sent |
-| Channel Name | The name of the Discord channel where the message should be sent |
+| Channel Name | Channel ID or channel name to send the message to |
 
 ### Outputs
 | Output | Description |