feat(payments): Add LangGraph integration for payment handling - #546

…path

Verify that when a raw-JSON tool returns 402 after error handler
recovery, the FallbackHandler is used to extract the real error detail
(e.g. 'budget exceeded') instead of falling through to 'unknown'.

When fallback detects a 402 in the recovery retry path, reassign _rh to
_FallbackHandler(fallback) so extract_body returns the actual parsed
body. Without this, GenericPaymentHandler looks for the PAYMENT_REQUIRED
marker, finds nothing in raw JSON, and the LLM sees 'unknown' instead
of the real error detail.

When both GenericPaymentHandler (marker) and _fallback_detect_402 (JSON)
fail to detect a 402, try the name-based handler from get_payment_handler
(e.g. HttpRequestPaymentHandler for tools named http_request). This
covers the legacy 'Status Code: 402' text-block format used by Strands
tools ported to LangGraph without format adaptation.

…hape

Custom handlers now receive result.content (the raw ToolMessage content)
for extract_status_code, extract_headers, and extract_body — not the
internal {'content': [{'text': ...}]} prepared shape.

This makes the custom handler contract intuitive: handlers parse the
tool's actual output format, not a middleware-internal wrapper. Fixes
silent detection failures when custom handlers expected raw JSON or
other native formats.

Add tests for:
- Async auto_session creation and reuse
- Async post-payment rejection with raw JSON fallback
- Async name-based handler fallback (legacy text-block format)
- Async custom handler receiving raw content
- Async error handler callback (retry and propagate paths)
- Sync custom handler raw content contract verification

Suite now at 141 passed, providing async path parity with sync tests.

- Merge _check_post_payment_rejection and _check_post_recovery_rejection
  into single _check_retry_rejection with context parameter
- Extract _inject_for_error_retry for shared injection in error handlers
- Extract _build_error_context for shared PaymentErrorContext construction
- Extract _handle_callback_resolution for shared resolution dispatch

Reduces middleware.py from 805 to 704 lines. The sync/async error
handlers now only differ at await/sleep/to_thread boundaries.
141 tests pass identically.

Move asyncio, json, GenericPaymentHandler, ErrorResolution, and
PaymentErrorContext imports to the top of the file. None have circular
dependency risks. Eliminates repeated imports that contributed to
drift between duplicated code blocks.

… contract

Update TrackingHandler and RawJsonHandler in test_functional.py to
handle raw content (str/list) instead of the prepared shape dict.
Required after the custom handler contract change in 076d6ed.