fix(metadata): tolerate absent/null jira, bugs, packages in JSONParser

[plusky]

JSONParser.parse iterated three metadata keys with no default:

for i in data.get("jira"): ...
for i in data.get("bugs"): ...
for prod, pkgvers in data.get("packages").items(): ...

If any key is absent (or explicitly null), .get() returns None and the
loop / .items() raises TypeError, aborting the whole parse. The current
metadata.json contract always carries these keys, so it doesn't fire today — but
a partial or malformed blob shouldn't crash the parser, and the sibling line
already does the right thing (data.get("repositories", [])).

Fix

data.get("jira") or [], data.get("bugs") or [], and
(data.get("packages") or {}).items() (handles both missing and null).

Test

test_json_parser_parse_tolerates_missing_optional_keys parses a minimal blob
(keys absent, packages null) and asserts empty results. Verified it fails on
the old code (TypeError) and passes on the fix.

Gates: ruff format/check clean, ty clean, pytest green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (d591e8f) to head (cbf19a6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #214   +/-   ##
=======================================
  Coverage   86.43%   86.43%           
=======================================
  Files         159      159           
  Lines        8832     8832           
=======================================
  Hits         7634     7634           
  Misses       1198     1198           

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