fix(indexer): keep typed source errors in IndexError

[lemarjohnny781]

Closes #111.

Problem

IndexError flattened its causes into String (Xdr, Strkey, Decode, and
Rpc), so the underlying typed error was gone by the time a caller saw it. The
only structure left was the coarse Http | Rpc split that is_transient used
for retry decisions — and Rpc(String) lumped together three genuinely
different situations: a JSON-RPC error object from the node, a simulation that
the node ran and rejected, and a response missing a field we need.

build_invoke_envelope was the worst case: map_err(|_| IndexError::Xdr(...))
discarded the xdr::Error entirely and replaced it with a hand-written string.

Change

• Xdr, Strkey, Decode carry their cause via #[from]
  (xdr::Error, stellar_strkey::DecodeError, serde_json::Error). The manual
  From<xdr::Error> for IndexError impl that stringified is removed, and seven
  .map_err(|e| ...e.to_string()) call sites collapse to ?.
• Rpc(String) splits into:
  • Rpc { code: Option<i64>, message: String } — the JSON-RPC error object,
    with the error code now captured instead of dropped;
  • Simulation(String) — the node ran the simulation and it failed;
  • MissingField(&'static str) — a well-formed response missing result or
    results[0].
• Envelope { context, source } replaces the ad-hoc Xdr(String) values built
  while encoding the invoke envelope, keeping both the typed xdr::Error and
  which part failed (function name / call arguments / operation list).
• UnsupportedAddress(ScAddress) holds the address instead of a Debug dump
  stuffed into Decode.
• is_transient is now an exhaustive match, so a new variant has to classify
  itself rather than silently defaulting to non-transient.

Behaviour

Deliberately unchanged.

• Retries: everything previously classified as Rpc — now Rpc,
  Simulation, MissingField — is still transient. Narrowing this (e.g.
  treating Simulation as permanent, since a trapped contract call will trap
  again) is now possible, but it's a behaviour change and doesn't belong in a
  typing fix.
• Logs: each Display string still embeds its cause, so existing
  error = %e output reads as before. Rpc additionally renders the code when
  the node sends one: rpc returned an error (code -32602): invalid params.

Tests

• Reworked only_http_and_rpc_errors_are_transient →
  only_transport_and_node_errors_are_transient, covering every new variant.
• Added errors_keep_their_typed_source: asserts Error::source() is present
  and downcasts to the original serde_json::Error / DecodeError /
  xdr::Error — i.e. the thing api: IndexError variants are stringly-typed (Xdr/Decode/Rpc hold String) — loses structured cause #111 asked for.
• Added rpc_error_display_includes_the_code_when_present.

Verification

cargo test (10/10 pass), cargo clippy --all-targets, and
rustfmt --check — the last shows no new diffs beyond the three that already
exist on main.

One note: main does not currently compile on its own — api/Cargo.toml is
missing rand, metrics and metrics-exporter-prometheus, several imports are
absent, AppState derives Clone over a non-Clone ArcSwap, and
routes::cache_headers calls a missing AppState::last_indexed_ledger. All of
that predates this branch and is untouched here; I stubbed it locally to get a
build for verification. Happy to send a separate PR for it if useful.

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