feat: fastswap intent top of block

[owen-eth]

Describe your changes

Adds an optional topPercentile int32 field to POST /fastswap. When set
(1-100), attaches a PositionConstraint{TOP, PERCENTILE, value} to the bid,
reusing the mechanism already used by the /top/percentile/N/ URL prefix.

Behavior:

• omitted or 0 -> no constraint, identical to today (non-breaking)
• 1-100 -> top-of-block constraint at that percentile
• out of range -> HTTP 400
• logged as topPercentile on swap submission

Scope:

• Intent path (POST /fastswap) only. Native-ETH path needs a separate
  rework (MetaMask broadcasts bypass our RPC).
• Bid amount is not currently adjusted when the constraint is set. Matches existing
  /top/percentile/N/ behavior. Possibly worth measuring commit rate before tuning.
• $100 min to be enforced client-side

Tests: TestHandleSwap_TopPercentile and TestHandler_TopPercentile
(valid / omitted / zero / out-of-range).

Second commit:

Pricer changes:

Extended Blocknative API call to fetch confidence level 90 (was 70/75/80/85, now 70/75/80/85/90). Added the corresponding switch case to store it.
Sender changes:

Two new constants: topOfBlockSwapConfidence = 80, topOfBlockSubsequentAttempts = 90
In calculatePriceForNextBlock: when a fastswap has a PositionConstraint set (txn.Constraint != nil && txn.Type == TxTypeFastSwap), override the confidence — p80 on first attempt, p90 on retries. This runs after the normal escalation logic, so it fully replaces the default 70/80/85 pattern with 80/90/90 for top-of-block swaps.

Issue ticket number and link

Fixes # (issue)

Checklist before requesting a review

• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation