Add bug_map as a cached property for LandingsRiskReportGenerator

[Karthikvenugopal]

Summary

Implements the follow-up from #5053: bug_map is now a @cached_property on
LandingsRiskReportGenerator instead of a local dict in go() passed as an
explicit argument to every method that needs it.

Changes

• Add from functools import cached_property.
• Add a bug_map cached property reproducing the existing filtering (keep bugs
  we are interested in, bugs blocked by other bugs, and bugs that caused
  regressions).
• Promote the locals the property depends on (bugs, test_info_bugs,
  meta_bugs) to instance attributes set in go() before bug_map is first
  accessed.
• Drop the bug_map parameter from get_blocking_of,
  generate_landings_by_date, generate_component_connections, and
  generate_component_test_stats, and update their bodies and the call sites in
  go() to use self.bug_map.

Why the dependencies had to move too

bug_map is filtered against bugs_set, which is derived from the bugs,
test_info_bugs, and meta_bugs locals computed earlier in go(). A
cached_property only receives self, so those inputs had to live on the
instance for the property to rebuild the same set. They are assigned in go()
before any consumer touches self.bug_map.

Notes

• No method mutates bug_map, so sharing a single cached dict across consumers
  is safe.
• The generator is instantiated once and go() runs once per process, so
  keeping these values on the instance does not change the effective memory
  lifetime.

Testing

• pre-commit run --all-files passes (ruff, ruff-format, mypy, codespell, etc.).
• No existing test exercises this script.

Fixes #5053

[Karthikvenugopal]

Hi @suhaibmujahid, this PR implements the @cached_property follow-up you suggested in #5053. Since bug_map is filtered against bugs_set (derived from the bugs, test_info_bugs, and meta_bugs locals in go()), I promoted those three to instance attributes so the property can build it from self alone; the four methods that previously took bug_map as a parameter now read self.bug_map. pre-commit run --all-files passes. Would appreciate your review when you get a chance. Thanks!