fix(rtds): serialize crypto_prices filters as comma-separated string (closes #270)

[HxrmxStudio]

Summary

Subscription::crypto_prices serializes its filters field as a JSON array (["btcusdt","ethusdt"]), but the Polymarket RTDS server expects a comma-separated plain string ("btcusdt,ethusdt") per the published schema. The server silently rejects the JSON-array body with {\"message\":\"Invalid request body\"} while leaving the WebSocket connection open — consumers see a live stream that never forwards any ticks, with no visible error anywhere in the SDK path.

This matches the bug described in #270. The reporter closed that issue without a fix merged, so v0.4.4 still ships the broken serialization. This PR adopts the reporter's one-liner recommendation.

Repro (observed in production)

Downstream consumer of polymarket-client-sdk = \"0.4.4\", subscribed via:

```rust
client.subscribe_crypto_prices(Some(vec!["btcusdt".into(), "ethusdt".into(), "solusdt".into(), "xrpusdt".into(), "dogeusdt".into()]))
```

Outcome: binance_ticks_forwarded counter stayed at 0 across ≥6 hours while chainlink_ticks_forwarded (on the same Client) accumulated at a healthy ~4.95 tps. No RTDS stream error warnings, no stream ENDED log line — the stream simply never yielded any items.

Passing None instead (no server-side filter) resumes tick flow immediately, confirming the filter shape is the root cause.

Fix

Replace the JSON array encoding with a join(\",\"):

```rust

• let filters =

•    symbols.map(|s| serde_json::to_string(&s).unwrap_or_else(|_| \"[]\".to_owned()));
  

• let filters = symbols.map(|s| s.join(","));
  ```

The existing custom Serialize impl already handles plain-string filters correctly via the fallback branch (the JSON re-parse fails for a comma-separated string and falls through to map.serialize_entry(\"filters\", filters)?, which emits the string value as-is). No changes needed there.

The two unit tests that encoded the old (broken) expected shape are updated to match the documented wire format:

• serialize_subscription_request — asserts \"filters\":\"btcusdt,ethusdt\"
• serialize_mixed_subscriptions — same assertion in the multi-topic case

Chainlink

crypto_prices_chainlink is unaffected. Its filter shape (\"{\\\"symbol\\\":\\\"btc/usd\\\"}\") is correct per #136 and remains serialized as an escaped JSON string by the existing Chainlink branch in the Serialize impl.

Test plan

• `cargo test --lib --features rtds` → 36 passed / 0 failed
• Updated assertions match the RTDS schema example in the official docs
• crypto_prices_chainlink serialization unchanged (verified by serialize_chainlink_subscription passing)
• Maintainer to verify against a live RTDS server

Closes #270.

---

Note

Low Risk
Low risk wire-format fix limited to RTDS Binance crypto_prices subscription serialization; main risk is behavior change for any consumers relying on the previous (incorrect) JSON-array encoding.

Overview
Fixes RTDS Binance crypto_prices subscriptions to serialize filters as a comma-separated string (e.g. "btcusdt,ethusdt") instead of a JSON array, matching the published RTDS schema and preventing silent server rejection/no-tick streams.

Updates unit tests to assert the new on-the-wire format (including mixed Chainlink+Binance requests) while keeping Chainlink filter serialization unchanged.

^{Reviewed by Cursor Bugbot for commit 223716d. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.52%. Comparing base (77264a4) to head (223716d).

Files with missing lines	Patch %	Lines
src/rtds/types/request.rs	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
- Coverage   85.54%   85.52%   -0.02%     
==========================================
  Files          32       32              
  Lines        5167     5168       +1     
==========================================
  Hits         4420     4420              
- Misses        747      748       +1     

Flag	Coverage Δ
rust	85.52% <66.66%> (-0.02%)	⬇️

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