fix(alpen-cli): reserve gas for alpen drain

[voidash]

Summary

• reserve Alpen drain gas using wide U256 arithmetic
• submit the drain transaction with the same explicit gas_limit and gas_price used for the balance calculation
• add focused tests for exact-fee, below-fee, and wide-arithmetic cases

Bug Encountered

During the local k2 alpen-cli smoke run, alpen drain --alpen-address 0x000000000000000000000000000000000000c0Fe failed after the CLI wallet had been funded and had already sent an Alpen transfer. The RPC rejected the drain transaction with:

insufficient funds for gas * price + value: have 998966468221608000 want 998979000000000000

The old code estimated gas for a 1-wei transaction, computed balance - gas_estimate * gas_price, then submitted a different max-value transaction while leaving the provider to fill gas fields again. That allowed the submitted transaction's upfront gas requirement to exceed the amount reserved by the drain calculation.

This PR fixes that by calculating the reserved fee with U256, returning early when there is no spendable balance after gas, and setting gas_limit and gas_price explicitly on the submitted drain transaction.

[github-actions]

Commit: 99c6d00

SP1 Execution Results

program	cycles	gas
EVM EE Chunk	824,732	969,395
EVM EE Account	404,056	498,593
Checkpoint	2,601,525	3,007,269

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.78%. Comparing base (55907ce) to head (e3b82c7).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
bin/alpen-cli/src/cmd/drain.rs	93.75%	4 Missing ⚠️

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HEAD has 2 uploads less than BASE

Flag	BASE (55907ce)	HEAD (e3b82c7)
functional	2	0

@@             Coverage Diff             @@
##             main    #1931       +/-   ##
===========================================
- Coverage   84.31%   69.78%   -14.54%     
===========================================
  Files         633      631        -2     
  Lines       75681    76312      +631     
===========================================
- Hits        63812    53254    -10558     
- Misses      11869    23058    +11189     

Flag	Coverage Δ
functional	?
unit	69.78% <93.75%> (+0.08%)	⬆️

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Files with missing lines	Coverage Δ
bin/alpen-cli/src/cmd/drain.rs	34.48% <93.75%> (+34.48%)	⬆️

... and 341 files with indirect coverage changes

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[storopoli]

ACK cc7c519

[barakshani]

We seem to have issues with gas calculation in general, not only in drain -- I'm not sure if this general issue is only when we haven't had sufficient activity (tx history is totally empty or in some recent block range). I think we just don't query what is the minimal gas the network accepts.
Shouldn't we try to fix it more holistically?

[voidash]

@barakshani

We seem to have issues with gas calculation in general, not only in drain -- I'm not sure if this general issue is only when we haven't had sufficient activity (tx history is totally empty or in some recent block range). I think we just don't query what is the minimal gas the network accepts.
Shouldn't we try to fix it more holistically?

This is only for drain specific invariant so i believe this is acceptable for narrow drain correctness fix. Activity history stuff is not the root issue
Before this pr we didn't set gas_limit(gas_estimate) and gas price. We sent with only to and value.
This would have been broken even if the chain had lots of activity or no activity.

But i agree that we need something on AlpenWallet that is a shared transaction gas helper.