Fix pending tips processing for verified Rewards profiles

[zenparsing]

Resolves brave/brave-browser#20090
Resolves brave/brave-browser#19982

In the current implementation, the pending tips processor will attempt to process records that are added while it is running. This can lead to an infinite (async) loop in the case of external wallet funded contributions. This change implements a minimal solution, where the start time of the process is recorded and compared to the added_date value of pending tip entries. Pending tips that are added after the "processor start time" are skipped.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

• Open browser with a new profile in staging.
• Enable Rewards.
• Verify with an external wallet provider (e.g. Uphold).
• Visit a site that is creator-verified in staging with a different wallet provider.
  • For example, if you linked with Uphold, visit http://github.com/miyayes (a Gemini-verified creator in staging).
• Make a one-time tip.
• Visit brave://rewards and view the list of pending tips.
  • Verify that the one-time tip created above appears in the list of pending tips.
• Trigger pending tips processing.
  • This can be accomplished by visiting the creator site above, opening the panel, and pressing the "Refresh" status button.
• Visit brave://rewards and view the list of pending tips.
  • Verify that the one-time tip created above appears in the list of pending tips, and no other tips appear.
• Close the browser.
• Modify the database to simulate a case where the creator has verified with a matching provider.
  • Open publisher_info_db.
  • Browse the pending_contribution table.
  • In the database row, change the publisher_id field value to the publisher ID of a publisher that is verified on a matching external wallet provider.
    • For example, if you linked with Uphold, use laurenwags.github.io.
  • Save the changes and close the database.
• Restart the browser.
• Trigger pending tips processing.
  • This can be accomplished by visiting the publisher used in the database step above (i.e. laurenwags.github.io), opening the Rewards panel, and pressing "Refresh".
• Visit brave://rewards and verify that the pending tip has been sent.

[emerick]

LGTM

[szilardszaloki]

++