Preserve explicit tab title override on settings reload

[ThanhNguyxn]

Summary of the Pull Request

• Preserve explicit runtime tab title overrides (for example from wt --title) when settings are reloaded.
• Add a regression test that reproduces the reload path and verifies the title remains stable.

References and Relevant Issues

Closes #19340

Detailed Description of the Pull Request / Additional comments

During settings reload, TerminalPaneContent::UpdateSettings refreshes controls from profile-based cached settings. That flow can overwrite a runtime title override with the profile default title.

This patch keeps explicit overrides by detecting when the current control starting title differs from the profile-derived starting title and then rebuilding control settings with a NewTerminalArgs title override for that pane.

Validation Steps Performed

• Added SettingsReloadPreservesExplicitTabTitleOverride in LocalTests_TerminalApp/TabTests.cpp.
• Ran git diff --check for touched files (clean).
• Attempted local diagnostics/build execution, but this environment lacks MSBuild (msbuild: command not found) and LSP startup is blocked on Windows+Bun 1.3.5 crash limitation.

PR Checklist

• Closes Windows title set with "--title" parameter gets reset to be "Default" after some time #19340
• Tests added/updated
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)