fix: parse remote authority from right

[EhabY]

Closes #937.

Summary

• Parse Coder remote authorities from the right, rejoining all safeHost segments before username/workspace extraction.
• Drop legacy empty-safeHost and --agent parser expectations; add double-dash/triple-dash safeHost regressions.
• Remove empty-hostname fallbacks in path/secrets/SSH config helpers and avoid session lookup when no deployment is selected.

Tests

• pnpm test:extension ./test/unit/util.test.ts ./test/unit/core/pathResolver.test.ts ./test/unit/remote/sshConfig.test.ts
• pnpm typecheck
• pnpm lint
• pnpm test:extension
• pnpm format:check

Implementation notes (Coder Agents generated)

• parseRemoteAuthority now requires coder-vscode.<safeHost> and at least username/workspace fields.
• The parser treats the last -- segment as workspace/agent, the previous segment as username, and rejoins earlier segments as safeHost.
• Tests cover Latin double-dash safeHost, triple-dash Punycode-like safeHost, and multiple embedded -- segments.
• Legacy deployment-unaware helpers and tests were removed per issue scope.

Generated by Coder Agents.