Bump install.sh / install.ps1 pin to unsloth>=2026.7.3

[danielhanchen]

Summary

• PyPI release unsloth 2026.7.3 is now live.
• Bumps the pinned floor in install.sh and install.ps1 from unsloth>=2026.7.2 to unsloth>=2026.7.3, and the paired unsloth-zoo>=2026.7.2 to unsloth-zoo>=2026.7.3, so fresh installs resolve to the new wheel.
• Updates all 5 install commands in each of install.sh and install.ps1 (reinstall / upgrade / no-torch / local / auto torch backend paths).
• Tagged on main as v0.1.49-beta.

Test plan

• Sanity-run install.sh on a clean Linux host
• Sanity-run install.ps1 on a clean Windows host
• Confirm uv pip install "unsloth>=2026.7.3" resolves cleanly

Follows the same pattern as #5716.

[gemini-code-assist]

Code Review

This pull request updates the minimum required version of unsloth and unsloth-zoo from 2026.7.2 to 2026.7.3 across various installation scenarios in both install.ps1 and install.sh. There are no review comments, and I have no feedback to provide.

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