♻️ refactor(logic): improve error handling and reduce duplication

[gaborbernat]

When users misconfigure the :module: or :func: directive options, the extension produced raw Python tracebacks (ImportError, AttributeError) that gave no indication of what went wrong or how to fix it. 🔐 This is especially confusing for new users who may not realize the error originates from their RST configuration rather than a bug in the extension.

The dynamic import is now wrapped with explicit error handling that surfaces clear messages through Sphinx's warning system, e.g. Failed to import module 'foo' or Module 'foo' has no attribute 'bar'. A subtle sys.modules leak on the AttributeError path was also fixed — when the module imported successfully but the function wasn't found, the stale module entry remained cached, which could cause the wrong parser to load in subsequent directive invocations within the same process.

♻️ The duplicated prefix resolution logic between _build_opt_grp_title and _build_sub_cmd_title has been consolidated into a shared _resolve_prefix method. Both methods had near-identical branching for combining title_prefix, sub_title_prefix, and program name, but diverged in small ways that made independent maintenance error-prone. A redundant _strip_ansi_colors call was also eliminated — _mk_sub_command already strips ANSI from parser.prog before passing it to _build_sub_cmd_title, which was stripping it again.

The coverage threshold has been raised from 76% to 100% to match actual coverage and prevent silent regressions. Additional test fixtures exercise previously untested argparse features (nargs variants, choices, action="count", action="append", required=True) and both error paths for bad module/function references.