test(solana): gate plain text at its 7.15 capability

[BitHighlander]

Summary

• run the plain-text Solana message test from firmware 7.15, where the capability ships
• reset the device before both AdvancedMode-negative message tests so live-session policy cannot leak across test order
• keep the binary-message refusal test active on older supported firmware

Multi-release contract

This targets the canonical reconcile/upstream-sync branch. Release behavior remains selected by firmware version/capability gates; no release-specific python-keepkey lineage is introduced.

Validation

• python3 -m py_compile tests/test_msg_solana_signtx.py
• git diff --check

Copilot review has not been requested.