Retry a rejected report grouping instead of falling back

[shellygr]

Fixes #211.

A cloud run produced a report whose coverage.warnings began with FALLBACK GROUPING APPLIED, collapsing all 11 formalized properties into the single
general bucket. The cause:

report: grouping failed: 1 validation error for GroupingResult
groups
  Input should be a valid list [type=list_type, input_value='{"groups":[...]}', input_type=str]

The model serialized the whole GroupingResult document into the one field
meant to hold the list. The grouping was complete and correct; only the
encoding was wrong, and it landed in the same bucket as a transport failure.

What changes

A string that parses as the grouping is decoded, not rejected. A
field_validator(mode="before") on GroupingResult.groups handles both shapes
seen and plausible: the whole document ({"groups": [...]}) and a bare list.
Anything that does not decode is returned untouched so pydantic raises the same
error it always did. This is parsing rather than a fallback, so a usable answer
is not thrown away over its envelope.

Grouping gets the corrective retry the ranker already has.
call_grouping_llm previously made one structured call and let any schema miss
reach build's fallback. It now retries once with the rejection appended,
mirroring rank_properties in composer/spec/prioritize.py. The correction is
appended to the original request rather than sent as a second turn, so each
attempt stays one self-contained user message.

The single-bucket fallback is unchanged and still covers a grouping that could
not be obtained at all.

Tests

Five added. Each half of the fix is covered by a test that fails without it:
neutralizing the decoder fails both string-wrapping tests, and dropping
max_attempts to 1 fails the retry test.

• a string-wrapped document decodes
• a string-wrapped bare list decodes
• a string that is not a grouping is still rejected
• the retry fires once, carries the rejection, and the first attempt does not
• after the retry it raises, so the caller's fallback still applies

1145 pass, pyright clean.