fix(poller): put transaction fetching behind a source trait

[ayomidearegbeshola29-dev]

What

Routes poll_contract through a new TransactionSource trait, with
HorizonSource as the only implementation today, so a Soroban RPC backend can
be added later without touching the polling logic.

Also surfaces Horizon's error body, which was being discarded.

closes #4

Why

Horizon's /accounts/{id} routes only accept G-addresses. A watched contract
(C…) address is rejected with HTTP 400, so the current single source cannot
actually poll a contract. An RPC source is the fix, but it is not a drop-in:
RPC can only enumerate the events a contract emitted, not the transactions it
took part in, so a reverted invocation is invisible to it, and there is no
pagingToken to advance a cursor with. Putting the seam behind a trait first
means that work is additive.

This PR does not fix C-address polling. It makes the limitation a
documented, tested property of one implementation rather than a hard-wired
assumption in the poller.

Error bodies

Horizon explains a rejection in the body, not the status line. That body was
being thrown away, so a rejected contract address logged only HTTP 400 —
indistinguishable from any other 400, and hard to diagnose from the field. It
now reports the offending field and reason:

Horizon returned HTTP 400 Bad Request for https://horizon-testnet.stellar.org/accounts/C…/transactions?…: account_id: Account ID must start with `G` and contain 56 alphanum characters

A rejected contract is counted as a poll failure rather than aborting the
cycle, so the operational symptom is a watch that silently never sees anything.

Also included

An earlier uncommitted fix rode along: include_failed=true on the
transactions request (#2). Without it Horizon omits failed transactions and the
TransactionFailed rule can never fire.

Tests

• horizon_c_address_rejection_names_the_account_id_field (unit) — pins the
  status and field name from the recorded Horizon response.
• horizon_rejects_a_contract_address_on_the_accounts_endpoint (integration) —
  replays the recorded 400 and asserts the contract is counted as a poll
  failure, with no transactions or alerts.

cargo test --workspace and workspace clippy are clean.

Follow-up

fetch_soroban_details discards its error body the same way. Left alone here
to keep this change scoped.

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