Fix ChibiOS virtual serial short packet receive

[giri256]

Description

ChibiOS virtual serial currently asks the USB OUT queue for a full CDC_EPSIZE buffer. The queue copies short CDC packets into that buffer, but the existing boolean receive helper returns false unless the requested size was filled exactly. As a result, normal short host writes are consumed and silently discarded before virtser_recv() is called.

This change adds a byte-count receive helper and keeps the existing exact-size boolean wrapper for Raw HID and MIDI. virtser_task() now dispatches exactly the bytes that were received, matching the LUFA implementation.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• Fixes [Bug] virtser_recv appears inoperative #26326

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

Verification

• Formatted the touched files with QMK's repository .clang-format configuration.
• git diff --check passes.
• Confirmed Raw HID and MIDI retain exact-size receive behavior through the existing wrapper.
• Confirmed the new virtual-serial path processes only the byte count returned by the ChibiOS input queue.

The first-time QMK container image pull stalled locally, so the authoritative ChibiOS compile result is left to upstream CI.