test: cover reachable coverage gaps

[retr0h]

Closes the four coverage gaps that are actually reachable. Zero-count statement blocks: 12 → 8.

The remaining 8 are deliberate and stay untouched — four defence-in-depth validation.Struct() calls that CONTRIBUTING tells contributors to keep, and four branches the code itself documents as unreachable (process/debian.go "gopsutil always succeeds", agent/seed.go "default body untestable", tracing/tracer.go the empty no-exporter case). 8 is the floor unless those conventions change.

What was covered

• internal/validation — the account_name validator closure, via validation.Var rows for valid names, digits, hyphen, underscore, trailing $, exactly 32 characters, 33 characters, uppercase, leading digit, leading hyphen, slash, space and empty; plus its customHints entry, by triggering a failure and asserting the message carries the pattern and the maximum.
• internal/telemetry/process — the memory-info error branch.
• internal/agent/handler.go — the ErrJobEnvelopeMissing arm of the signature error switch. Its siblings (bad signature, not enrolled) were already covered; this one was not.

One production change, and why

self.go needed more than a test. gopsutil's MemoryInfo() does not fail in practice, and the existing SetMemoryInfoFn seam replaces the whole closure, so it can never exercise the real one. The raw call now sits behind its own procMemoryInfoFn, matching the cpuPercentFn style beside it, and the default is a named defaultMemoryInfoFn so ResetMemoryInfoFn restores that exact function rather than a duplicate closure — Go tracks coverage per source position, so a duplicate left the block looking uncovered. Behaviour is unchanged.

Context: main was at 99.8% and failing its gate after two security fixes added untested guards; #514 restored it, and this closes what remains reachable.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FuKUsHFG1EqZXamffh9M2c

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
+ Coverage   99.90%   99.95%   +0.04%     
==========================================
  Files         489      489              
  Lines       23179    23180       +1     
==========================================
+ Hits        23157    23169      +12     
+ Misses         22       11      -11     

Files with missing lines	Coverage Δ
internal/telemetry/process/self.go	100.00% <100.00%> (+5.00%)	⬆️

... and 2 files with indirect coverage changes

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