Jetpack Post by Email feature: numeric-only tags are not processed correctly

### Impacted plugin

Jetpack

### Quick summary

Numeric-only tags (e.g., `[tags 3322]`) are not processed correctly in the Post by Email feature. Alphanumeric tags (e.g., `[tags pp3322]`) work as expected.

### Steps to reproduce

1. Enable the Post by Email feature
2. Compose an email and include a numeric tag (e.g., `[tags 3322]`).
3. Send the email to be published.
4. Check the post once published and see that the tag is not displayed
5. Compose another email
6. Add an alphanumeric tag this time (e.g., `[tags pp3322]`) and send it.
7. Check the post published and see that the tag was correctly processed and displayed

### A clear and concise description of what you expected to happen.

Issue was reported by a user in the forum, here: https://wordpress.org/support/topic/jetpack-post-by-email-tag-shortcode/ but I was able to replicate this on my test site.

### What actually happened

_No response_

### Impact

Some (< 50%)

### Available workarounds?

No but the platform is still usable

### If the above answer is "Yes...", outline the workaround.

_No response_

### Platform (Simple and/or Atomic)

_No response_

### Logs or notes

_No response_

Start by locating the Jetpack Post by Email feature and its tag shortcode processing, then reproduce the issue with [tags 3322] and [tags pp3322]. The work is done when numeric-only tags are processed and displayed in published posts just like alphanumeric tags.

Written by the indexing model from the issue text.