gh-407: Mark optional platform tags in online aliases

[Punisheroot]

Summary

• py list --online now renders the default platform suffix as optional in the Alias column (python[w]3[-64].exe), matching the Tag column (3.15-dev[-64]).
• Before, the default install showed plain python3-64.exe while the -32 install correctly showed python3[-32].exe.

Context

Each online feed entry lists both the bare alias (python3.exe) and its
platform variant (python3-64.exe), but format_table only displays the
first occurrence of each name. The -32 entry comes first in the feed and
claims the bare names, leaving the default -64 entry with lone suffixed
names that were rendered without brackets.

Fixes #407

Changes

• src/manage/list_command.py: _format_alias now receives the command and forwards default_platform to get_install_alias_names.
• src/manage/installs.py: get_install_alias_names accepts default_platform=None (default preserves existing behavior for the install-command callers); a lone suffix equal to the default platform renders as [<plat>].
• tests/test_list.py: added test_format_table_aliases_default_platform, reproducing the exact table from Inconsistent optional platform tags in list --online #407 with real feed alias shapes.

Validation

• pytest tests/test_list.py tests/test_installs.py: PASS (30 passed; new test fails without the fix with the exact output from the issue)
• pytest tests/test_list.py tests/test_installs.py tests/test_install_command.py tests/test_commands.py: PASS (63 passed)
• git diff --check: PASS
• Full suite: 450 passed, 7 failed — all 7 reproduce on the clean tree (5x BITS download WinError -2145386386, network-restricted sandbox; plus test_get_current_package and test_IdleCommand, environment-dependent). Unrelated to this change.

Compatibility and risk

• Display-only change to the table formatter; CSV/JSON/id/exe formats untouched.
• Non-default single suffixes (-32, -arm64) render exactly as before; cmd=None callers keep the old behavior.
• No dependency, config, or lockfile changes.