Re-running setup no longer collapses more than one services host (#891)

[Ninja-FSE]

Fixes #891.

What was wrong

Found reviewing #811's merged change (ca79f53). configure.py's services-host prompt showed the first configured ADMIN_HOSTMASKS entry as its default and, on a blank answer, fell back to it - then always wrote one entry:

admin_host = input(f"Your services host (blank to skip){suffix}: ").strip() or default_host
...
if admin_host:
    changes["ADMIN_HOSTMASKS"] = [f"*!*@{admin_host}"]

An operator with two hosts configured (home and phone, say) who re-runs configure.py and presses Enter at that prompt has the list collapsed to the first. The second silently loses admin access to the console and the in-channel commands.

A smaller oddity in the same step: if the first existing entry is itself a wildcard pattern (*!*@*.home.net), the default becomes *.home.net, and pressing Enter printed "That will not do: '*.home.net' has a '*' in it" about a value the operator never typed.

What changed

• A blank answer now writes nothing - ADMIN_HOSTMASKS is left exactly as it is, rather than rewritten from the first entry.
• A host actually typed still replaces the list, and now says so first when more than one was already configured: "This replaces the 2 services hosts already configured with just this one."
• The default shown is only offered when it would itself pass admin_host_problem() - a wildcard first entry is no longer offered, so Enter never runs the validator against a value the operator didn't type.

Test

tests/test_configure.py (3): a blank answer with two hosts configured writes nothing (fails on the old code with the exact reported shape - collapsed to the first entry alone); a typed answer that replaces more than one says so, checked against the printed text; a wildcarded first entry is not offered as the default and prints no confusing refusal. test_the_console_guide_says_what_configure_does.py's anchor updated to check the actual defect (no fallback on the blank answer) rather than an exact source string this change moved.

Verified

Full suite in a clean clone: 6408 OK (58 skipped).

🤖 Generated with Claude Code

[chchatzop]

This duplicates #903 (Fixes #891, opened yesterday, green) - and #903 is the base of a stacked chain (#904 -> #905 -> #906), so which one lands matters beyond this issue. Both fix the reported case (Enter writes nothing). Two differences, both on the side of losing a host, which is what #891 is about:

1. Typing a host that is already configured still wipes the others. With home and phone set, re-running and typing the home host out of habit replaces both with just home - the phone loses admin. The "This replaces the 2 services hosts..." line prints after the host is typed, with no way to back out. #903 treats a host already present as no change, and adds a new host beside several rather than replacing them (one question cannot know which of several a new host replaces).
2. The comma-separated form is indexed as characters. adminchat.admin_host_patterns() accepts ADMIN_HOSTMASKS as a string ("*!*@a,*!*@b"); current_hostmasks[0] is then "*", and len(current_hostmasks) would announce "replaces the 13 services hosts". #903 reads the hosts the way the console does (admin_host_patterns()), and shows them (Configured now: ...).

Suggest merging #903 and closing this one - it keeps the chain above it intact, and its tests cover both cases (test_a_rerun_of_configure_keeps_every_admin_host.py, 8). If you prefer this one, it needs those two before it is safe, and I will rebase the chain onto whatever lands.