feat(signal): Add SignalContext for strategy signals and catalyst facts

[huacnlee]

Adds a standalone SignalContext covering the three signal APIs.

What you get

Method	Endpoint	Purpose
signals(opts)	GET /v1/signals	Query signals — filter by symbol, strategy, catalyst and time range; page with limit / offset. Returns the page plus a total.
signal(id)	GET /v1/signals/{signal_id}	One signal, including the full strategy analysis in json_data.
security_facts(opts)	GET /v1/facts/security_facts	A security's fact (catalyst) events — anomaly detections, factor readings, sources and summaries.

A signal is a strategy's take on a security, triggered by a catalyst — it carries a title, a Markdown summary, an outlook, a risk level and conservative / benchmark / optimistic target prices, and names the fact that triggered it in key_fact_id.

Notes on the modelling

• symbol, not counter_id. The response carries counter_id, stock_symbol and symbol with the same value; only symbol is exposed.
• Outlook is an enum. All 250 signals available on staging map onto exactly five labels, matching the core_conclusion.outlook_enum scale 1..=5 inside json_data: Strong bullish, Bullish, Neutral, Bearish, Strong bearish. Serialization round-trips back to the wire label.
• Time-range parameters are OffsetDateTime, following GetHistoryExecutionsOptions, and serialize as RFC3339 (start_time=2024-01-15T10:30:00Z).
• Millisecond timestamps. created_at / updated_at arrive as millisecond epoch strings, which the existing second-resolution serde_utils::timestamp cannot parse, so a timestamp_ms helper was added next to it.
• risk_level and status stay untyped — staging shows only "" / "R4" for the former and a constant 1 for the latter, which is not enough to pin down either domain. Happy to turn them into enums once the value sets are confirmed.
• json_data stays a string. The per-signal analysis document is strategy-specific and runs to ~10 KB; it is carried verbatim rather than modelled.
• Facts are raw JSON objects. The payload is fact-type specific (news / fundamental / technical) with different fields per type.

Verification

signals and signal were verified against staging (LONGBRIDGE_ENV=staging) — real responses deserialize, round-trip, and produce RFC3339 timestamps; the query builder emits the documented parameter names.

Two things could not be verified there and are worth a second pair of eyes:

• GET /v1/facts/security_facts returns {"facts": []} on staging for every symbol tried (including symbols that have signals, with and without a time range), so only the envelope is confirmed — not the item shape.
• On staging the signals filters (symbol_name, strategy_id, strategy_name, catalyst_name, catalyst_type, start_time, end_time) appear to be ignored — total and the returned rows are unchanged even for a nonexistent symbol. limit / offset do work. The SDK sends all of them as documented.

Scope

Only the Rust SDK (async + blocking). The Python / Node.js / Java / C bindings are not included — happy to follow up with those.

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