fix: map deepseek and z.ai to openai-compatible-model in platform alias

[nudiltoys-cmyk]

Related Issue

Closes #1578
Closes #1556

Description

both issues report BYOK validation failing with "unknown model platform: "
deepseek falls through PLATFORM_ALIAS_MAPPING because there's no entry, so camel's ModelPlatformType receives the bare string "deepseek" and raises
z.ai is currently mapped to "zhipu" which camel also doesn't recognize
both providers expose openai-compatible apis and src/lib/llm.ts already sends the platform strings as-is, so the fix is two entries in the mapping: change z.ai from zhipu to openai-compatible-model, and add deepseek mapped to the same target
matches the existing pattern used by grok, ModelArk, and llama.cpp

discussed first on #1578
the issue author proposed exactly this fix in the body, and i confirmed the change locally before opening the pr

Testing Evidence (REQUIRED)

ran the full backend suite per .github/workflows/test.yml:

$ uv sync
$ uv run pytest tests/app -q
403 passed, 11 skipped, 30 warnings in 10.52s

ran ruff against the whole backend per .github/workflows/pre-commit.yml, version pinned to match .pre-commit-config.yaml (ruff-pre-commit v0.14.3):

$ ruff --version
ruff 0.14.3
$ ruff check .
All checks passed!
$ ruff format --check .
142 files already formatted

targeted run on the touched test file:

$ pytest tests/app/model/test_model_platform.py -v
tests/app/model/test_model_platform.py::test_normalize_model_platform_maps_known_aliases PASSED
tests/app/model/test_model_platform.py::test_normalize_model_platform_keeps_non_alias_unchanged PASSED
tests/app/model/test_model_platform.py::test_normalize_optional_model_platform_handles_none PASSED
tests/app/model/test_model_platform.py::test_normalized_model_platform_type_applies_in_pydantic_model PASSED
4 passed in 0.03s

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline