docs: langchain parity table (Python ↔ TypeScript)

[devin-ai-integration]

Summary

The Python half of the cross-language parity documentation for the new @keycardai/langchain package (typescript-sdk#158). README-only; no code changes, and the title is unscoped so nothing releases.

Adds a "Contract parity with TypeScript" section to packages/langchain/README.md: a row per contract concept (middleware, identity factories, accessor, resources/scopes, credential, sign-in and authorization URLs, fallback identity, escape hatch, testing seam, error accessors, ungranted read, interrupt payloads, attempt cap) with Python spelling vs TypeScript spelling, plus the three deliberate divergences — the TS escape hatch is a callback (so there is no agrant), the TS middleware owns its contextSchema, and MCP composition stays Python-only in v1.

Every row was verified against both sources rather than assumed, including the error paths: get_resource_error / getResourceError, ResourceAccessError on ungranted reads, the subject_token_expired vs missing_identity sign-in reasons, and the shared 3-attempt acquisition cap.

Link to Devin session: https://app.devin.ai/sessions/3c9fc9cb3a1143f6a9d21f172a366682
Open in Devin Desktop: https://app.devin.ai/desktop/session/3c9fc9cb3a1143f6a9d21f172a366682?variant=devin
Requested by: @Larry-Osakwe

[devin-ai-integration]

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