refactor: prevent future RpcError variants from breaking downstream matches

[suchapalaver]

Summary

Marks the public RpcError enum #[non_exhaustive] so that future
operational variants (further per-endpoint health/quota signals,
follow-ons to the ConflictingRateLimit variant added in this
release, etc.) can be added without forcing a major-version bump on
every downstream consumer. The cost is one-time: exhaustive matches
over RpcError written in a downstream crate need a wildcard
(_ => ...) arm. Aligns the discipline already applied to the peer
public provider-surface types RpcConfig and ChainEndpoint.

Closes #62.

What's in the diff

• src/errors/rpc.rs — adds #[non_exhaustive] to the pub enum RpcError
  declaration plus a # Stability rustdoc section explaining the
  contract to downstream consumers.
• CHANGELOG.md — new bullet under [Unreleased]/Breaking Changes
  spelling out the wildcard-arm requirement and the forward-compatibility
  benefit, framed in operator-facing language and linking to the
  asymmetry it resolves with the peer types.

Acceptance check (from #62)

• #[non_exhaustive] added to pub enum RpcError (src/errors/rpc.rs).
• Breaking change documented under [Unreleased]/Breaking Changes,
  noting the _ => ... requirement.
• Landed in the same minor bump as the existing breaking changes,
  not spent as a standalone bump.

Test plan

• cargo fmt --check clean
• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo test — 97/97 pass (default features)
• cargo test --all-features — 97/97 pass
• cargo test --no-default-features — 97/97 pass
• In-crate exhaustive matches over RpcError: none (verified by
  grep across src/, tests/, examples/). The only match on a
  type named RpcError lives in src/transport/retry.rs:363 but
  binds alloy's alloy_json_rpc::RpcError, a name collision with
  our type.

[suchapalaver]

Self-review pass

Pre-commit /code-review + post-PR /pr-review, with findings triaged below.

#	Finding	Status
1	New # Stability rustdoc heading on RpcError (src/errors/rpc.rs:83-92) diverges from how the peer #[non_exhaustive] types document the attribute — RpcConfig (src/config/policy.rs:65-66) and ChainEndpoint (src/provider/pool.rs:490-491) use a single inline sentence at the tail of the existing rustdoc, no heading.	Skipped — defensible divergence. For an enum, the wildcard-arm requirement is a contract a downstream consumer needs to discover before writing a match; a dedicated heading aids that. The struct cases are lighter (only break un-..'d struct literals) and the inline-sentence treatment is appropriately quieter. Project-wide rustdoc convention for #[non_exhaustive] documentation is not worth a follow-up issue — future authors can copy whichever neighbour is closest.

No blockers remaining. All three feature combos (cargo test, --all-features, --no-default-features) green; clippy clean under --all-targets --all-features -- -D warnings; commit signed. The breaking change is documented under [Unreleased]/Breaking Changes so it lands in the same minor bump as the other entries there rather than spending another bump.