feat(grpc): migrate MDDS endpoints onto in-house transport and remove tonic

[userFRM]

Follow-up to #522 / #524.

Goal

PR #524 lands the in-house gRPC transport (Channel, Codec, Status,
ServerStreaming), hardening (deadlines, GOAWAY classification,
cancellation), and ChannelPool. This issue tracks the remaining
migration work: get every MDDS endpoint onto the in-house transport
and remove the tonic / tonic-prost / tonic-prost-build deps
entirely.

Scope

Custom codegen replacing tonic-build

• New build-time step under crates/thetadatadx/build_support/grpc/
  invokes prost-build for message types only, then a custom
  ServiceGenerator emits one async function per RPC method.
• Output file: \$OUT_DIR/grpc_endpoints_generated.rs, include!'d
  from crates/thetadatadx/src/grpc/endpoints.rs.
• Each generated function:

  pub async fn <method_snake_case>(
      channel: &Channel,
      req: <RequestType>,
  ) -> Result<ServerStreaming<<ResponseType>>, ChannelError>

• Idempotent regeneration. Add a build-script --check mode
  (env-var-gated) for CI drift detection.

Migrate every MDDS endpoint

• mdds/macros.rs list_endpoint! and parsed_endpoint! macros
  switch from self.stub().$grpc(request).await to the generated
  crate::proto::beta_theta_terminal::$grpc_snake(&self.channel, request).await.
• mdds/client.rs swaps the tonic::transport::Channel field for
  grpc::ChannelPool (MddsClient::connect builds a 4-channel pool
  by default; each call picks the next channel).
• mdds/stream.rs collect_stream and for_each_chunk switch from
  tonic::Streaming<ResponseData> to
  grpc::ServerStreaming<ResponseData>.
• Retry / refresh loops in the macros update their error
  classification: ChannelError::DeadlineExceeded →
  StatusClass::Transient, ChannelError::ConnectionClosed →
  StatusClass::Transient (with channel recycle),
  ChannelError::Rpc { status } → existing from_tonic_status
  logic ported to read grpc::Status::code.

Remove tonic

• Drop tonic / tonic-prost / tonic-prost-build from
  crates/thetadatadx/Cargo.toml.
• Remove the From<tonic::Status> impl and the GrpcStatusKind
  mapping in error.rs; replace with From<grpc::Status> reading
  the numeric code directly.
• After the migration, cargo tree -i tonic returns empty for the
  workspace.

Drop the inhouse-grpc feature flag

• Once the in-house path is the only path, the feature flag has no
  variants to gate. Optional deps (h2, http, http-body,
  bytes, pin-project-lite, futures-core) move to required.

Update the bench

• benches/grpc_channel.rs switches to in-house-only and adds 2-3
  representative endpoints beyond stock_list_symbols
  (stock_history_eod, option_history_quote).

Constraints

• Conventional Commits, factual messages.
• TDD where the change is testable through the macros' surface
  (deadlines, retry on DeadlineExceeded, retry on
  ConnectionClosed).
• No #[allow(dead_code)], no TODO / FIXME / XXX in shipped code.
• cargo fmt --all -- --check, cargo clippy --workspace --locked -- -D warnings, cargo test --workspace --locked,
  cargo run -p thetadatadx --features config-file --bin generate_sdk_surfaces --locked -- --check,
  python3 scripts/check_docs_consistency.py — all green.
• Cross-binding ABI stays unchanged — Python / TypeScript / C++
  bindings keep working without recompile.

Definition of done

• All 60+ MDDS endpoints route through grpc::ChannelPool ->
  grpc::Channel::server_streaming.
• cargo tree -i tonic is empty.
• The inhouse-grpc Cargo feature is gone (or relabeled if needed
  for downstream consumers).
• Bench numbers reported in the PR description for at least three
  representative endpoints.

[userFRM]

Folded into PR #524 — codegen + endpoint migration + tonic removal all ship there.