[FPF] Refactor ScriptBlockingRuleApplierService

Refactor the script blocking rule application logic to improve clarity
and robustness.

The previous implementation had multiple states representing the absence
of rules (nullopt, empty string, invalid JSON), making the logic
complex.

This change introduces a clearer state management by using std::optional
throughout the data flow. A std::nullopt now unambiguously signals that
the content rule list should be removed.

The service's logic is split into BuildRules() and BuildAndApplyRules()
to separate concerns. ContentRuleListData and the component installer
are updated to propagate the optional state.

This prepares the code for adding UMA metrics.

Bug: 418218469
Change-Id: I081dd3ef2b51fc5e3aff5ff8e139ce944558008b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6867492
Reviewed-by: Martin Pan-Verde <thesalsa@google.com>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Ron Sailer <rsailer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1504586}