perf(bg/monetization): reuse PaymentSession when link added

[sidvishnoi]

Context

Part of #976
The goal is to efficiently find the minSendAmount for a PaymentSession.

Changes proposed in this pull request

Reuse PaymentSession instead of starting a new one.

Original PR summary

• In the first commit (e4a8ae5), it won't have the desired effect, as we have an AbortSignal that makes the probing restart.
• In the next commit (c5de59f), we replace adjustAmount(rate) with findMinSendAmount() (which is independent of rate). Extracted into 1048.
• When a new link tag is added, we want to keep using older minSendAmount, even if the adjustAmount may change due to splits. adjustAmount resulting rate is >= minSendAmount, so it makes sense to find minSendAmount first, and not find it ever again.
  • Currency fluctuations can change this minSendAmount in future, but then we'd need to probe/findMinSendAmount again (existing bug, will fix some time later).

Also updated the constructor type for PaymentSession a bit. Moved to #1047. I was thinking of adding a static PaymentSession.from(), but we don't need it right now. The new constructor format will help there as well.

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Before (see quotes requests increasing as link tags added to page, including for existing tags):

Screen.Recording.2025-05-01.at.19.13.12.mov

After (see quotes requests for link tags already added are skipped; fewer requests):

Screen.Recording.2025-05-01.at.19.09.40.mov

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[sidvishnoi]

test-e2e
Edit: Unrelated tests fail (wallet budget, flaky), so all good.