perf(asgi): avoid quadratic request body aggregation and add explicit ASGI body cap #70

Context

Current ASGI bridge request-body read path aggregates chunks using repeated bytes concatenation (body += chunk), which causes repeated realloc/copy under fragmented payloads. It also has no explicit ASGI-side cap before full aggregation.

This can create avoidable CPU/memory pressure and is a practical DoS vector for large or highly chunked bodies.

Current code

• oxyroute/asgi.py in asgi_to_rsgi(...):
  • read loop appends chunks to body (bytes)
  • no ASGI-side max-body guard

Goals

1. Replace repeated bytes concatenation with an O(n) accumulation strategy.
2. Introduce explicit ASGI-side max-body limit (aligned with existing body-size policy).
3. Return a clear error/termination behavior on limit breach.

Proposed direction

• Accumulate into bytearray or list[bytes] and join once.
• Enforce max bytes while reading chunks.
• Keep behavior consistent with framework-level 413 handling semantics where possible.

Acceptance criteria

• No body += chunk pattern in ASGI bridge hot read path.
• Payload limit is enforced during read (not only after full aggregation).
• Add tests for:
  • normal chunked body path;
  • over-limit chunked body path.
• Existing ASGI tests continue to pass.

Out of scope

• Full streaming request-body API for handlers (separate issue).