Transform paths and repeat joining for response parsing (#47648)

Add dotted transform paths and join mode to response templates

Transform placeholders can now take dotted paths ({content.args}) to
extract keys from parsed dicts, so tool-call envelopes whose key names
differ from the standard format can be reshaped without custom code.
Repeated fields can set join (a separator string) to concatenate their
matches into one string instead of collecting a list, for models that
emit several thinking/text blocks per message. Includes an Inkling
(TMLv0) template fixture exercising both.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>