fix: handle precondition-failed error type in document PUT/POST responses

[chomatdam]

Summary

• put-response and post-response in the document routes now handle :com.yetanalytics.lrs.xapi.document/precondition-failed errors by returning HTTP 412 (Precondition Failed)
• This enables downstream implementations (lrsql) to return structured error maps from -set-document when If-Match / If-None-Match validation fails, and have them correctly surfaced as 412 responses

Context

The existing etag validation in etags-preproc runs in a separate interceptor stage before the actual document write. This creates a TOCTOU (Time-of-Check-Time-of-Use) race condition — between the etag check and the write, another request can modify the document, causing silent overwrites despite correct use of concurrency headers.

To fix this, the downstream LRS implementation needs to perform atomic check-then-write within a single DB transaction. This PR provides the error-handling plumbing so that a precondition failure detected at write time is correctly propagated as a 412.

Changes

• put-response: Check ex-data for ::precondition-failed type → 412
• post-response: Add ::precondition-failed → 412 branch (using cond instead of nested if for clarity alongside existing 400 error types)

Companion PR

Requires corresponding changes in lrsql (atomic etag validation in -set-document) to produce the :com.yetanalytics.lrs.xapi.document/precondition-failed error type:
yetanalytics/lrsql#504

Testing

Tested via lrsql make test-sqlite:

• xAPI 1.0.3 conformance: 1365/1365 passed
• xAPI 2.0.0 conformance: 1435/1435 passed
• Unit tests: 88 tests, 923 assertions, 0 failures, 0 errors

[milt]

This is a subset of the work @cliffcaseyyet mentions we need to do here yetanalytics/lrsql#504 (comment), the error name is sensible and the handling is correct so I'm going to approve + merge this and then add the rest of the applicable actions and routes. Thanks again @chomatdam !