Add test for showInteractiveDialogue's no-options fallback

[dmccoystephenson]

Summary

• Full-repo triage this cycle found no open issues/PRs and no drift: schemas match their models and *JsonReaderWriters, README/PLANNING.md/Dockerfile/requirements.txt/install scripts all match actual behavior, money/price displays are correctly %.2f (or legitimately always-integer, using %d), no test relies on unseeded random, and no test hits a real filesystem/display.
• Coverage review (pytest --cov=src) showed src/ui/baseUserInterface.py at 82%, the lowest of any module. The uncovered non-abstract lines were showInteractiveDialogue's fallback branch for an NPC with zero currently-available dialogue options (e.g. every option's condition is false) — this is shared logic both the pygame and web front-ends inherit without override, so it's real production behavior, just never exercised by a test.
• Added test_inherited_interactive_dialogue_falls_back_to_introduction_when_no_options, characterizing the current (correct) behavior: the NPC's introduce() text is shown via showDialogue instead of an empty question menu.

Per the entire-untouched-backlog note: there was no backlog to defer — zero open issues and zero open PRs at triage time, and no functional bugs were found during the review, so this cycle's only outcome is the test-expansion work above (Phase 2 Stage B).

Test plan

• python3 -m compileall -q src
• Full suite (pytest, headless SDL) — 464 passed, up from 463
• Coverage on src/ui/baseUserInterface.py rose from 82% → 88%; all remaining uncovered lines are unreachable @abstractmethod pass stub bodies

[dmccoystephenson]

Self-review rubric:

• Scope: PASS — only tests/ui/test_baseUserInterface.py is touched; this is a pure test-expansion (Stage B) cycle, no production code changed.
• Tests-new: PASS — test_inherited_interactive_dialogue_falls_back_to_introduction_when_no_options exercises BaseUserInterface.showInteractiveDialogue's previously-untested "zero available dialogue options" branch (baseUserInterface.py:119-122).
• Tests-fix: N/A — this cycle adds characterization coverage, not a bug fix; no production code was changed so there's nothing to stash-and-revert.
• Sibling structure: PASS — follows the # prepare/# call/# check comment convention and the existing FakeNPC/ConditionalNPC fixture-class pattern already used in this file; the new NoOptionsNPC fixture mirrors them.
• Sibling renames: N/A — no renames in this diff.
• Docs: PASS — no persisted fields or user-facing behavior changed; nothing in the Phase 7 doc sources needs updating.
• Issue resolution: N/A — no open issues existed at triage time (Phase 1 found zero open issues, zero open PRs), so there is no Closes #N and no backlog was deferred.
• CI: PASS — green on the PR head SHA (gh pr checks 137 --watch).

Repo-specific:

• Schema-sync: N/A — no persisted field changes.
• Money-format: N/A — no money/price display touched.
• Deterministic-tests: PASS — no random usage in the new test.
• Headless-pygame: N/A — baseUserInterface.py has no pygame dependency (that's pygameUserInterface.py).
• camelCase: PASS — new test/class naming matches this file's existing conventions (test_snake_case functions per pytest convention already used throughout, PascalCase fixture classes, get_dialogue_options/introduce method names matching the real NPC/BaseUserInterface interface signatures).

Coverage on src/ui/baseUserInterface.py rose from 82% → 88% (pytest --cov=src); all remaining uncovered lines are unreachable @abstractmethod pass stub bodies, which can never execute directly. Full suite: 464 passed (up from 463), 0 failed.

No open issues or PRs existed at Phase 1 triage, so no backlog was deferred this cycle.