* models: add extra_payload to create_custom / update_custom

Mirrors the new atlas-app backend field (LayerLens/stratix-app#1932).
``extra_payload`` is an optional JSON object that's deep-merged into
every outgoing chat-completions request body — customer values win on
conflict with our hardcoded defaults. Lets callers override
``temperature`` (we send 0 for reproducible evals) or add
provider-specific fields like ``max_completion_tokens`` that some
endpoints require.

Updated:

- ``models.create_custom`` (sync + async) — new ``extra_payload``
  keyword argument.
- ``models.update_custom`` (sync + async) — same, plus a docstring
  note that ``{}`` clears the existing payload.
- Reference docs, README example, samples, CHANGELOG.
- Example ``api_url`` values now show the full ``/v1/chat/completions``
  form to match the backend hint.

Existing SDK callers keep working — the field is optional and additive
on the backend.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* test(models): cover extra_payload in create_custom / update_custom

Five new cases exercising the new parameter:
- create_custom forwards extra_payload verbatim
- create_custom omits the key when None
- update_custom supports extra_payload-only updates
- update_custom({}) sends an empty dict so the backend clears the
  stored payload (semantics added in atlas-app#1932)
- update_custom omits the key when None

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore: bump version to 1.6.2

Releases the ``extra_payload`` parameter added in the previous commits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore: re-bump version to 1.7.0

The previous commit bumped to 1.6.2, but adding the new
``extra_payload`` parameter counts as new functionality, so the
semver-correct minor bump is 1.7.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>