fix(scripts): fall through to grep/sed when python3 is a broken stub in feature.json parser

[Quratulain-bilal]

fixes #3304

what

read_feature_json_feature_directory in scripts/bash/common.sh chose its JSON parser by availability:

if command -v jq ...; then ...
elif command -v python3 ...; then ...   # stub passes this
else grep/sed fallback fi               # unreachable once python3 exists

on windows python3 usually resolves to the microsoft store app execution alias stub. it satisfies command -v python3 but fails at runtime (exit 49, "Python was not found..."). the elif selected it, the failure was swallowed to _fd='', and the grep/sed last resort was never reached. so a valid .specify/feature.json read as empty, and every script that resolves a feature this way (setup-plan.sh, setup-tasks.sh, check-prerequisites.sh, ...) errored with Feature directory not found right after a successful specify init --script sh — as reported.

how

select by parse success, not availability: try jq, then python3 only if the result is still empty, then grep/sed only if still empty. a parser that exists but yields nothing now falls through instead of ending the chain.

the write path (_persist_feature_json) already uses jq-or-printf with no python3, so it wasn't affected; only the read path needed this.

verified

reproduced end-to-end with git-for-windows bash, a stub python3 (exit 49) first on PATH, and no jq:

• before: bash setup-plan.sh --json → ERROR: Feature directory not found (exit 1)
• after: same command → resolves the feature and writes plan.md (exit 0), emits the expected JSON paths

added test_setup_plan_survives_broken_python3_stub: writes a python3 stub that exits 49, prepends it to PATH, and asserts setup-plan.sh still succeeds via the grep/sed fallback. it uses the same @requires_bash harness as the existing tests in that file (skips where a suitable bash isn't present).