Propagate dynamic BYOC capability prices

[its-DeFine]

Summary

• Store BYOC external capability prices as *AutoConvertedPrice handles instead of *big.Rat snapshots.
• Wire RegisterExternalCapability() to preserve the dynamic price handle created from price_per_unit, price_scaling, and currency.
• Add regression tests proving oracle/feed updates change advertised BYOC prices without re-registering the capability.
• Add BYOC gateway-token coverage proving two registered external capabilities return distinct dynamic prices through /process/token, and both update after a feed change.

Refs #3929.

This is the first working slice for per-capability oracle pricing: BYOC prices now remain live after registration and the gateway-visible token path sees the current price for each requested external capability. It does not yet add separate per-capability oracle/feed selection fields.

Verification

Run inside dedicated PC2 VM go-livepeer-byoc-usdc-20260509 with Go 1.26.3 and Livepeer-patched FFmpeg libavfilter 10.1.100:

go test ./byoc -run TestGetToken_GatewaySeesDistinctDynamicBYOCPrices -count=1 -v
go test ./byoc -count=1
go test ./core -run TestBYOCExternalCapsDynamicPriceUpdates -count=1

Passed:

=== RUN   TestGetToken_GatewaySeesDistinctDynamicBYOCPrices
--- PASS: TestGetToken_GatewaySeesDistinctDynamicBYOCPrices (0.01s)
PASS
ok  github.com/livepeer/go-livepeer/byoc  0.031s
ok  github.com/livepeer/go-livepeer/byoc  0.080s
ok  github.com/livepeer/go-livepeer/core  0.037s

Previous focused coverage also passed in the same VM:

go test ./core -run 'TestBYOCExternalCaps(DynamicPriceUpdates|PriceInfo|SenderPricing|PriceEdgeCases)|TestNewAutoConvertedPrice|TestAutoConvertedPrice_Update|TestExternalCapability_GetPrice' -count=1
go test ./byoc -count=1

Also checked broader go test ./core ./byoc -count=1; ./byoc passed, while ./core hits existing base-branch failure TestPriceInfo_GivenNilRecipient_ReturnsNilError. Verified the same focused failure exists on base commit a78e876ff6c97d935344643c2547d0f84d4b26dd, so it is not introduced by this PR.