fix(go-sdk): surface json.Marshal errors in memory backend requests

[ardittirana]

Fixes #434.

Problem

mustJSONReader discarded the json.Marshal error:

func mustJSONReader(v any) io.Reader { data, _ := json.Marshal(v); return bytes.NewReader(data) }

When a value can't be serialized (unsupported type, cyclic struct, channel, func), json.Marshal returns nil, err, the error is dropped, and the function returns a reader over a nil slice. The downstream control-plane POST is then sent with an empty body — silently storing nothing or overwriting with an empty value — and the real error never reaches the caller, making data-loss bugs in the memory backend very hard to debug.

Fix

Replace it with jsonReader(v any) (io.Reader, error) that returns the marshal error, and propagate that error at each of the five call sites (Set, Get, Delete, SetVector, SearchVector) — all of which already return an error. No behavior change on the success path. Stdlib only.

Test

Updates the existing helper test and adds a case asserting that an unserializable value (a channel) returns an error and a nil reader instead of silently yielding an empty body. go test ./agent/ passes (all existing backend tests included); gofmt/go vet clean.

[CLAassistant]

All committers have signed the CLA.

[santoshkumarradha]

Thanks @ardittirana, this looks good.

CLA is still pending. Can you sign it here?

https://cla-assistant.io/Agent-Field/agentfield?pullRequest=679

Once that clears, we can continue.

[ardittirana]

Signed 🫡

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Go	221 B	-21%	0.61 µs	-39%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 84%, aggregate ≥ 85%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.10%	87.40%	↓ -0.30 pp	🟡
sdk-go	91.70%	92.00%	↓ -0.30 pp	🟢
sdk-python	93.73%	93.73%	↑ +0.00 pp	🟢
sdk-typescript	90.20%	90.42%	↓ -0.22 pp	🟢
web-ui	84.94%	84.79%	↑ +0.15 pp	🟡
aggregate	85.74%	85.75%	↓ -0.01 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	0	—	➖ no changes
sdk-go	31	67.00%	❌
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

❌ Patch gate failed

sdk-go — 67.00% on 31 touched lines (10 uncovered):

File	Patch coverage	Missing lines
sdk/go/agent/control_plane_memory_backend.go	67.7%	61, 62, 94, 95, 135, 136, 217, 218, 306, 307

How to fix

1. For each file listed above, add tests that exercise the missing line numbers in this same PR.
2. Re-run locally: ./scripts/coverage-summary.sh && ./scripts/patch-coverage-gate.sh.
3. Do not lower min_patch in .coverage-gate.toml to silence this — the floor is the contract.