refactor(prefs): single-source-of-truth Firefox prefs dict + obsolescence verifier in update.py

[vringar]

Summary

Makes OpenWPM's default Firefox preferences a single source-of-truth that both
the setter (configure_firefox.py) and the obsolescence verifier read,
so the two are structurally interlocked and cannot drift. Wires the verifier
into scripts/update.py so every dependency/Firefox update auto-checks the
prefs we set for obsolescence.

Stacks on #1140 (fix/obsolete-firefox-prefs). This refactors the
already-cleaned pref set from that PR; merge #1140 first. The diff shown
against master will look larger until #1140 lands.

What changed

1. Prefs consolidated into dicts (single source of truth)

openwpm/deploy_browsers/configure_firefox.py now defines two module-level
dicts:

• OPTIMIZE_PREFS: dict[str, bool | int | str] — the 54 static
  speed/telemetry/safebrowsing/update prefs, with the original comment grouping
  preserved. optimize_prefs() is now for name, value in OPTIMIZE_PREFS.items(): fo.set_preference(name, value).
• PRIVACY_PREFS: dict[str, bool | int | str] — the privacy pref names
  (privacy.donottrackheader.enabled, network.cookie.cookieBehavior). These
  prefs' values are dynamic (computed from browser_params), so privacy()
  still sets them inline/conditionally; only the names live in the dict so the
  verifier can probe them.

Behavior is identical — same prefs, same values, same ordering (privacy
then optimize), and browser_params.prefs still override last. Verified by
mock-capturing the new optimize_prefs() output and diffing it against the
pre-refactor source: 54/54 optimize prefs match on name+value, privacy
set_preference calls byte-identical, and PRIVACY_PREFS covers exactly the
privacy pref names.

2. Verifier moved + interlocked

datadir/verify_obsolete_prefs.py → scripts/verify_obsolete_prefs.py. It
now imports PRIVACY_PREFS and OPTIMIZE_PREFS and probes their
.keys() — no hardcoded/copied pref list. It keeps the Firefox
default-branch probe (getDefaultBranch('').getPrefType) and the honest
caveat: a default-branch absence is a strong-but-not-absolute signal, so
INVALID prefs are reported as review candidates, not auto-removed. Takes
FIREFOX_BINARY from the environment.

3. Wired into scripts/update.py

After the Firefox bump step (firefox_version.update_if_needed()),
check_obsolete_firefox_prefs() runs scripts/verify_obsolete_prefs.py
against the installed Firefox (firefox-bin/firefox-bin, or $FIREFOX_BINARY)
and prints a clear WARNING block listing any pref that now probes obsolete.
Fully non-fatal: a missing binary, a launch failure, or obsolete prefs only
warn — the update is never blocked.

Verification

• from openwpm.deploy_browsers.configure_firefox import PRIVACY_PREFS, OPTIMIZE_PREFS → 2 54
• import openwpm.deploy_browsers.deploy_firefox imports cleanly
• scripts/verify_obsolete_prefs.py imports and builds a 56-pref list (privacy-first, no dupes)
• Applied pref-set proven unchanged vs. the pre-refactor source (see above)
• update.py skip path exercised (non-fatal when no binary present)
• pre-commit (black / isort / mypy) green

A full crawl was not run (browser env-blocked); the static "same prefs" proof
is the key check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.40%. Comparing base (1986eb2) to head (1b73290).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           fix/obsolete-firefox-prefs    #1201      +/-   ##
==============================================================
- Coverage                       61.87%   61.40%   -0.48%     
==============================================================
  Files                              40       40              
  Lines                            3892     3842      -50     
==============================================================
- Hits                             2408     2359      -49     
+ Misses                           1484     1483       -1     

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