byoc: support offchain mode without an Eth keystore

[rickstaa]

Summary

Fixes #3905. BYOC currently requires -ethPassword=... on both gateway and orchestrator even in -network offchain mode — a throwaway keystore exists only to satisfy a vestigial signing requirement. After this PR, BYOC inherits transcoding's existing offchain story: no keystore, no -ethUrl, just -network offchain.

Root cause (recap from #3905)

Both halves of the signing flow are already offchain-aware:

• core/broadcaster.go:14 — Sign() returns []byte{}, nil when node.Eth == nil.
• core/orchestrator.go:66 — VerifySig() returns true when node.Eth == nil.

BYOC's wrapper code prevented that bypass from being reached:

1. byoc/job_gateway.go — getJobSender always prefixed "0x", producing the literal string "0x" for empty sigs.
2. byoc/job_orchestrator.go — verifyTokenCreds only stripped "0x" when len(Sig) > 130, so the literal "0x" passed through to hex.DecodeString and failed.

Changes

• byoc/job_gateway.go — extracted encodeJobSig() that returns "" for empty sigs and "0x" + hex otherwise.
• byoc/job_orchestrator.go — verifyTokenCreds unconditionally trims a "0x" prefix via strings.TrimPrefix. Empty sig now decodes to empty bytes, VerifySig short-circuits to true offchain. Also fixed a missing format verb in the existing Unable to hex-decode signature log line.
• Tests — TestEncodeJobSig (helper), TestVerifyTokenCreds_OffchainEmptySig (round-trip), TestVerifyTokenCreds_StripsHexPrefix (regression for short prefixed sigs).

Test plan

• go test ./byoc/... passes (existing + new tests)
• go build ./byoc/... clean
• Manual E2E: BYOC docker-compose with -network offchain on both nodes, no -ethPassword, no -ethUrl. Capability registers, /process/request/<suffix> round-trips through gateway → orchestrator → worker container. (Will validate in livepeer-python-gateway hello-world example once this lands.)

Context

Surfaced while building a Pipeline SDK hello-world test against unmodified go-livepeer. The example currently uses -ethPassword=secret-password as a workaround — once this lands, the compose drops the keystore flag entirely.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 32.92286%. Comparing base (9e68815) to head (c8e674f).

Files with missing lines	Patch %	Lines
byoc/job_orchestrator.go	50.00000%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3906         +/-   ##
===================================================
+ Coverage   32.87450%   32.92286%   +0.04836%     
===================================================
  Files            171         171                 
  Lines          42063       42065          +2     
===================================================
+ Hits           13828       13849         +21     
+ Misses         27194       27173         -21     
- Partials        1041        1043          +2     

Files with missing lines	Coverage Δ
byoc/job_gateway.go	44.08946% <100.00000%> (+0.72376%)	⬆️
byoc/job_orchestrator.go	49.45946% <50.00000%> (+0.80355%)	⬆️

... and 4 files with indirect coverage changes

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Files with missing lines	Coverage Δ
byoc/job_gateway.go	44.08946% <100.00000%> (+0.72376%)	⬆️
byoc/job_orchestrator.go	49.45946% <50.00000%> (+0.80355%)	⬆️

... and 4 files with indirect coverage changes

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