fix(playground): standardize RPC env and post-rename demo/tooling fixes

[EfeDurmaz16]

What

Cross-language playground fixes, grouped because they all surfaced while getting the playground demo working end to end.

1. Standardize the RPC override on PAY_KIT_RPC_URL

Python and Ruby read PAY_KIT_RPC_URL; TS and Go read the bare RPC_URL. A generic RPC_URL set for another service could silently redirect the backend's RPC and settle on a different fork. Move the TS and Go backends (and the Go CI step) onto the namespaced PAY_KIT_RPC_URL so every backend reads one knob. Per @lgalabru's review.

2. Restore the session receipt route (post-#193 leftover)

flow.ts polled /__402/payment-channels/receipt/:id, but both servers serve /sessions/receipt/:id and the Vite proxy only forwarded /__402, so the session demo never surfaced its on-chain settle signature. Point the client back at /sessions/receipt and add the /sessions proxy entry.

3. Send a metered POST body so the upto demo shows real billing

The endpoint workbench sent no request body, so the x402 upto summarize gate always metered 0 bytes (summarizedBytes: 0) and settled the 1-token minimum, making upto look like a flat charge. Send a sample text body for POST endpoints (summarize is the only one), so the demo shows per-byte usage-based billing.

4. Fix the Python coverage target (post-#193 leftover)

python/Justfile ran pytest --cov=pay_kit after the package was renamed to solana_pay_kit, measuring 0% and tripping the 90% gate. Point it at solana_pay_kit.

Test

Go playground builds; Python tests green; the rest verified against the running playground.

Read order

1. typescript/examples/playground-api/index.ts, the env rename at the reference implementation
2. go/examples/playground-api/main.go, same rename mirrored in Go
3. .github/workflows/go.yml, CI boot env renamed in lockstep
4. the two playground-api READMEs, env tables following the code
5. playground/app/src/lib/flow.ts, receipt poll repointed to the live /sessions/receipt/:channelId route
6. playground/app/vite.config.ts, the dev proxy enabling that poll (scoped to ^/sessions/)
7. playground/app/src/components/EndpointWorkbench.tsx, sample POST body so the summarize demo meters real bytes
8. python/Justfile, coverage target renamed to the actual solana_pay_kit package

Generated vs handwritten

No generated files; ~30 handwritten changed lines across 10 files, all reviewable in one pass.

Reviewer note

Everything cross-checks against all three playground server implementations, and the response shape of the receipt route ({settledSignature, finalized}) was verified in TS, Go, and Python. The one place that bit us was the vite proxy: a bare /sessions prefix shadowed the SPA's own Sessions page on refresh; fixed in 050ca12 by scoping to ^/sessions/.

Fable 5 self-review

The env rename and receipt-route fix are done properly, CI updated in the same commit. But the first cut shipped a proxy rule that broke refresh on the very page whose poll it fixes, despite the ^/x402/ precedent on the adjacent line: a copy-the-pattern miss, caught in re-check and fixed (050ca12) along with the last stale RPC_URL row in playground/README.md. SAMPLE_POST_BODY quietly assumes every POST endpoint wants prose; true today across all three APIs, worth gating on the endpoint id if a JSON POST endpoint ever lands.

[greptile-apps]

Greptile Summary

This PR fixes several playground demo and tooling issues.

• Standardizes playground RPC overrides on PAY_KIT_RPC_URL.
• Updates Go CI and playground docs for the renamed RPC setting.
• Restores session receipt polling through the /sessions/receipt route.
• Sends a sample POST body for the metered endpoint demo.
• Points Python coverage at the renamed package.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
go/examples/playground-api/main.go	The Go playground now reads the namespaced RPC setting.
typescript/examples/playground-api/index.ts	The TypeScript playground now reads the namespaced RPC setting.
python/examples/playground_api/app.py	The Python playground comment now matches the shared RPC setting.
.github/workflows/go.yml	The Go workflow now passes the namespaced RPC setting to the playground server.
playground/app/src/lib/flow.ts	Receipt polling now targets the session receipt route.
playground/app/vite.config.ts	The dev proxy now forwards session receipt API calls.
playground/app/src/components/EndpointWorkbench.tsx	POST endpoint runs now include a sample text body for metering.
python/Justfile	Coverage now targets the renamed Python package.

_{Reviews (4): Last reviewed commit: "fix(playground): scope the sessions dev ..." | Re-trigger Greptile}

[lgalabru]

Hmm, let's do this the other way, and get rid of RPC_URL ?

[EfeDurmaz16]

Good call. Flipped it the other way: the TS and Go backends (and the Go CI step that fed RPC_URL) now read the namespaced PAY_KIT_RPC_URL, which Python and Ruby already used. The bare RPC_URL is gone, so an ambient value can no longer redirect the playground RPC.