chore: release v0.2.0

…port

Add ResponsesRequestTranslator, ResponsesResponseTranslator, and
ResponsesStreamParser to bridge canonical Chat Completions format and
the OpenAI Responses API (/v1/responses). The layer handles message
→ input items, system → instructions, tool definitions, and full SSE
event translation including reasoning summaries.

ResponsesRequestConfig exposes backend-specific behaviour:
- SystemHandling enum (ExtractToInstructions | MapToDeveloper) to choose
  between public API conventions and the Codex private backend
- drop_max_tokens / drop_temperature / drop_top_p for ChatGPT OAuth
  sessions that reject these fields
- default_store / default_include / default_parallel_tool_calls /
  default_reasoning_effort / default_reasoning_summary defaults
- force_instructions to emit empty "instructions" when none provided
- max_tool_name_len for backends that enforce name limits

ResponsesRequestConfig::codex() preset matches CLIProxyAPI's Codex
behaviour out of the box so downstream proxies (e.g. BYOKEY) can
adopt the shared translator in one line.

Field mappings added:
- reasoning_effort shorthand and reasoning object → reasoning.{effort,summary}
- response_format (text / json_object / json_schema) → text.format
  with json_schema field flattening per Responses API spec
- parallel_tool_calls from extra with config fallback
- ResponseUsage input/output_tokens_details → canonical Usage.extra
  (prompt_tokens_details / completion_tokens_details)

Stream parser additions:
- response.reasoning_summary_text.{delta,done} → ReasoningDelta with
  "\n\n" separator between multi-segment summaries
- response.output_item.{added,done} for reasoning items → buffered
  encrypted_content emitted as ReasoningSignature, flushed before
  any subsequent tool_call
- Usage details mapped in response.completed events

aigw-core StreamEvent gains ReasoningDelta(String) and
ReasoningSignature(String) variants to carry reasoning model output
through the canonical event stream.

…098, 0099)

Co-authored-by: arcbox-labs[bot] <254083426+arcbox-labs[bot]@users.noreply.github.com>