perf(kv): skip merge of absent context/user/tenant maps in snapshot

[samber]

Summary

• mergeMaps allocates a fresh map even for empty input, so every snapshot() (called once per ToMap/LogValue/formatVerbose) paid up to three map allocations for context/userData/tenantData fields the error chain never set.
• Consumers only ever check len() on these fields, so leaving them nil when nothing was merged is equivalent.
• ToMap's in-place "id" insertion into s.userData/s.tenantData now guards against the nil case explicitly.

Benchmarks

benchstat (darwin/arm64, Apple M3, -count=6 -cpu=1):

            before          after
ToMap       5.120Ki ± 0%    5.073Ki ± 0%  -0.92% (p=0.002 n=6)
LogValue    5.264Ki ± 0%    5.217Ki ± 0%  -0.89% (p=0.002 n=6)
ToMap       62.00 allocs    61.00 allocs  -1.61% (p=0.002 n=6)
LogValue    60.00 allocs    59.00 allocs  -1.67% (p=0.002 n=6)

Wall-clock deltas were not statistically significant on this run (background noise on the benchmark machine), but the B/op and allocs/op reductions are fully reproducible (0% variance, p=0.002).

Test plan

Verified with go build ./..., go vet ./..., and go test ./....

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.24%. Comparing base (9be8af4) to head (cfded4d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
error.go	0.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   90.43%   90.24%   -0.20%     
==========================================
  Files          15       15              
  Lines        1234     1250      +16     
==========================================
+ Hits         1116     1128      +12     
- Misses         93       95       +2     
- Partials       25       27       +2     

Flag	Coverage Δ
unittests	90.24% <60.00%> (-0.20%)	⬇️

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[Copilot]

Pull request overview

This PR reduces per-call allocations in the internal snapshot() path by avoiding unnecessary map merges when the error chain contains no context/user/tenant maps, while preserving existing outward behavior for ToMap, LogValue, and verbose formatting.

Changes:

• Skip mergeMaps/lazyMapEvaluation/dereferencePointers for context/userData/tenantData when no maps were collected during snapshotting.
• Update OopsError.ToMap() to safely handle nil s.userData / s.tenantData when only the corresponding ID was set, before inserting "id".

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
kv.go	Avoids map merge/allocation work in snapshot() when no context/user/tenant maps exist in the chain.
error.go	Guards ToMap()’s in-place "id" insertion against nil snapshot user/tenant maps.

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