fix(workflows): resolve negative list indices in expressions

[NgoQuocViet2001]

Problem

_resolve_dot_path matches the index bracket with ^([\w-]+)\[(\d+)\]$, which accepts digits only. A negative index therefore never enters the indexing branch — it falls through to the dict lookup and asks for the literal key "task_list[-1]", which is absent, so the whole path resolves to None:

ctx = StepContext(steps={"tasks": {"output": {"task_list": [{"file": "a.md"}, {"file": "b.md"}]}}})

evaluate_expression("{{ steps.tasks.output.task_list[0].file }}", ctx)   # 'a.md'
evaluate_expression("{{ steps.tasks.output.task_list[-1].file }}", ctx)  # None

list[-1] is valid in both Python and the Jinja2 subset the module documents itself as providing, and "the last item a step produced" is a natural thing for a workflow template to want. There is no error: the template renders empty, and evaluate_condition on the same path reads false, so a step can be skipped for a reason that never surfaces.

Fix

Accept the negative form in the pattern and bound the index from both ends. Out-of-range in either direction keeps returning None rather than raising, matching the existing behaviour for [9] on a short list.

Scope

Two lines in src/specify_cli/workflows/expressions.py plus a docstring note, and one test next to the existing test_list_indexing. Positive indices and non-index path segments are untouched.

Test plan

• Ran: pytest tests/test_workflows.py -k "indexing or literal" → 11 passed.
• Ran: pytest tests/test_workflows.py → 942 passed. The 20 failures are the TestWorkflowCliAlignment symlink cases, which fail identically on an unmodified checkout here (Windows, no symlink privilege).
• Checked: reverting only expressions.py fails the new test with assert None == 'b.md'.

[Copilot]

🟡 Changes recommended

Condition-remediation parsing still rejects negative indices supported by the resolver.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds Python-style negative list indexing to workflow expressions.

Changes:

• Supports bounded negative indices.
• Adds resolution and bounds tests.

File summaries

File	Description
expressions.py	Extends list-index parsing and bounds checks.
test_workflows.py	Tests negative and out-of-range indices.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Balanced

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[mnriem]

Please address Copilot feedback

[NgoQuocViet2001]

Addressed — the Copilot finding was correct, thanks.

_PATH_SEGMENT and the indexed-root fullmatch were both still on \d+, so a condition the resolver handles was classified unresolvable and the correction withheld:

item[0] == 'x'   -> Wrap the expression: "{{ item[0] == 'x' }}".
item[-1] == 'x'  -> No correction is offered because 'item[-1]' is not a name
                    the evaluator can resolve

Both now allow -?\d+, and the two negative cases are in the existing parametrize beside test_an_indexed_item_root_keeps_the_correction. The refusal side is unchanged — inputs[0] and steps[-1] are still rejected as non-roots.

pytest tests/unit/test_condition_expression_block.py → 337 passed. Reverting only expressions.py fails the two new cases.