feat(tx-cache): add SSE subscription methods for transactions and orders

[Evalir]

Description

Add subscribe_transactions() and subscribe_orders() methods to TxCache behind an sse feature flag. These connect to the /transactions/feed and /orders/feed SSE endpoints exposed by tx-pool-webservice and return typed Streams of TxEnvelope / SignedOrder that terminate on first error.

Uses eventsource-stream as a lightweight SSE parser over reqwest byte streams — no HTTP client coupling, minimal deps (futures-core, nom, pin-project-lite).

Changes

• Workspace Cargo.toml: Add eventsource-stream = "0.2.3" to workspace deps
• tx-cache/Cargo.toml: Add sse feature gating eventsource-stream, serde_json, and reqwest/stream
• tx-cache/src/error.rs: Add Sse and Deserialization error variants with From impls, all behind #[cfg(feature = "sse")]
• tx-cache/src/client.rs: Add subscribe_inner<T> generic helper and two public async methods (subscribe_transactions, subscribe_orders)

Design decisions

• Terminate-on-error: Stream yields the error as its final item then ends — matches the existing stream_* convention. Reconnection can be layered by the consumer.
• Feature-gated: Consumers that don't need SSE pay no dep cost.

Related Issue

Closes ENG-1987

Testing

• Tests pass locally
• New tests added (if applicable)
• Clippy clean with --all-features and --no-default-features
• cargo +nightly fmt clean

[Evalir]

• feat(tx-cache): add SSE subscription methods for transactions and orders #209 👈 (View in Graphite)
• main

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