Fix executable bank-transfer examples in the contributor tutorial

[dajiaohuang]

Summary

Correct the existing examples in docs/contributing.rst, as proposed in #209 and approved by csingley on 2026-10-16 at 14:45:13 UTC:

• Import SubAggregate from ofxtools.Types and correct its documented location.
• Point INTRARQ.xferinfo at XFERINFO, and INTRATRNRQ.intrarq at INTRARQ.
• Restrict the response mutex to the corresponding intra* fields.
• Correct ofxtools.Type to ofxtools.Types, the bank message-set module path, and message-set bases to Aggregate.
• Supply the missing imports used by the sequential examples (STATUS, RECURRINST, Bool, ElementList, DAYS, Integer, ListElement, Time).

The original snippets raise import/name errors or illustrate incorrect model relationships. This changes only the tutorial; production code, tests and message design are unchanged. The header/parser discussion in #208 and separate charset direction are not included.

Validation and limits

Reused historical validation from 2026-10-15 at base 7758d23e9923ae13bdb71761666f39ccd21faa3f: all 22 corrected Python blocks executed sequentially in a fresh namespace, and the tutorial-defined request/response wrappers round-tripped through production Aggregate.from_etree() using invented account identifiers. No real financial data or network operations were used.

No tests, tutorial execution, Sphinx build or CI were run during this docs-only extraction. Historical validation is not a claim about current upstream HEAD or current CI.

Related: #209.

[csingley]

Thanks @dajiaohuang