Python: bump package versions for 1.2.1 release (#5536)

* Python: bump package versions for 1.2.1 release

PATCH bump (1.2.0 -> 1.2.1) for the released cohort. The release window
covers two PRs, no new public APIs:

- agent-framework-core: prevent inner_exception from being lost in
  AgentFrameworkException (#5167)
- samples: add requirements.txt and .env.example to the a2a/ hosting
  sample for pip-based setup (#5510)

Per lockstep convention, all 21 beta packages stamp 1.0.0b260428 and all
3 alpha packages stamp 1.0.0a260428, regardless of per-package code
churn. Every non-core package floor on agent-framework-core is raised to
>=1.2.1 to keep cohort signaling consistent. Date stamp reflects the
local (Asia) cut date 2026-04-28.

* Python: silence pyright unknown-type warnings in hosted-env detection

`azure.ai.agentserver.core` is probed at runtime via `importlib.util.find_spec`
and is not a declared dependency. The existing `# pyright: ignore[reportMissingImports]`
suppresses the missing-import warning, but at `lowest-direct` resolution pyright
still reports the imported symbol (`AgentConfig`) and its members (`from_env`,
`is_hosted`) as unknown, breaking `validate-dependency-bounds-test` for
`packages/core`.

Extend the existing ignore to cover `reportUnknownVariableType` on the import
and `reportUnknownMemberType` on the call site so the bounds check returns to
green. Behavior is unchanged.

Latent since #5455 (shipped in 1.2.0).

* Python: raise agent-framework-gemini lower bound to google-genai>=1.65.0

The Gemini chat client references several `google.genai.types` symbols
(`FileSearch`, `ThinkingLevel`, `SearchTypes`, `McpServer`,
`StreamableHttpTransport`, plus call-site keyword args `mcp_servers` and
`search_types`) that are not present at the lower bound of `google-genai>=1.0.0`.
At `lowest-direct` resolution this caused `validate-dependency-bounds-test` to
fail for `packages/gemini` with eleven `reportAttributeAccessIssue` /
`reportUnknownVariableType` errors.

Walking the upstream `google.genai.types` API:
- `GoogleMaps`, `AuthConfig`: present from 1.40.0
- `FileSearch`: introduced in 1.49.0
- `ThinkingLevel`: introduced in 1.55.0
- `SearchTypes`, `McpServer`, `StreamableHttpTransport`: introduced in 1.65.0

Bump the lower bound to 1.65.0 — the minimum version that exposes every symbol
the package actually uses. Keep the `<2.0.0` upper cap unchanged. With this
bump `validate-dependency-bounds-test` passes for both lower and upper
resolution scenarios across all 27 workspace packages.

Latent since #4847 (Gemini package introduction in 1.1.0); aggravated by
subsequent feature additions that pulled in newer `types.*` symbols.

* Python: add dependabot bumps to 1.2.1 CHANGELOG

Catalog the 15 dependabot dependency updates that merged on `upstream/main`
between python-1.2.0 and the 1.2.1 cut window under a new Changed section:

- Workspace dev/runtime deps: `rich`, `prek`, `python-multipart`, `pyasn1`,
  `pytest` (ag-ui, devui, lab), `uv` (lab)
- Frontend deps: `vite` (devui, chatkit), `postcss` (devui, chatkit, handoff),
  `picomatch` (devui, handoff)

CHANGELOG-only — no source or pyproject.toml changes. PRs themselves merged
upstream independently of this release branch and will be brought in via the
PR merge.

python/packages/devui/pyproject.toml

@@ -4,7 +4,7 @@ description = "Debug UI for Microsoft Agent Framework with OpenAI-compatible API
 authors = [{ name = "Microsoft", email = "af-support@microsoft.com"}]
 readme = "README.md"
 requires-python = ">=3.10"
-version = "1.0.0b260424"
+version = "1.0.0b260428"
 license-files = ["LICENSE"]
 urls.homepage = "https://github.com/microsoft/agent-framework"
 urls.source = "https://github.com/microsoft/agent-framework/tree/main/python"
@@ -23,7 +23,7 @@ classifiers = [
   "Typing :: Typed",
 ]
  dependencies = [
-     "agent-framework-core>=1.2.0,<2",
+     "agent-framework-core>=1.2.1,<2",
      "openai>=1.99.0,<3",
      "opentelemetry-sdk>=1.39.0,<2",
      "fastapi>=0.115.0,<0.133.1",