Handle ASCII in send_unicode_string with keycodes

[nkgotcode]

Description

Updates send_unicode_string() to emit ASCII characters through QMK's existing send-string keycode path when the decoded character has a keycode mapping. Non-ASCII characters continue to use the configured Unicode input mode.

Fixes #25382

Types of Changes

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

Issues Fixed or Closed by This PR

• Fixes [Bug] send_unicode_string() is inefficient for ASCII characters #25382

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.
• N/A: My change requires a change to the documentation.
• N/A: 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).

[tzarc]

Let's use the actual keycode.

Suggested change

	if (code_point > 0 && code_point < 0x80 && pgm_read_byte(&ascii_to_keycode_lut[code_point]) != XXXXXXX) {
	if (code_point > 0 && code_point < 0x80 && pgm_read_byte(&ascii_to_keycode_lut[code_point]) != KC_NO) {

[zvecr]

The logic is also better encapsulated if its implemented within quantum/send_string/send_string.{h,c} as something like is_valid_send_char().