fix(a2ui): pin schema_manager file reads to UTF-8

[genisis0x]

Problem

A2UISchemaManager in autogen/agents/experimental/a2ui/schema_manager.py loads JSON and text files from disk through five call sites:

• the custom catalog file passed by the caller (line 105);
• _load_spec_json and _load_spec_text reading the upstream A2UI spec bundle (lines 228, 236);
• _load_version_json and _load_version_text reading the version-pinned schemas (lines 243, 251).

None of these passed an explicit encoding, so on platforms whose default locale.getencoding() is not UTF-8 — notably Windows, which defaults to cp1252 / cp932 — non-ASCII bytes in catalog descriptions, component labels, or rule files raise UnicodeDecodeError mid-load and A2UISchemaManager.__init__ fails before the manager can be constructed.

JSON is defined over UTF-8 (RFC 8259 §8.1), and the .md rule files follow the same convention.

Change

• autogen/agents/experimental/a2ui/schema_manager.py — pass encoding="utf-8" at every file-read call site. No behavioural changes.
• test/agents/experimental/a2ui/test_schema_manager.py — new regression test test_custom_catalog_file_load_handles_non_ascii writing a catalog with Chinese ("智能按钮") and accented Latin ("Composant interactif — étiquette") characters across both the catalog $id and a nested component description, then asserting the loaded schema round-trips bit-for-bit.

Same pattern as the previously-merged UTF-8 encoding pins (#2815, #2818, #2819, #2825, #2826, #2827) and the matching pins in #2909 / #2910.

Validation

• uv run python -m pytest test/agents/experimental/a2ui/test_schema_manager.py -v — 15/15 pass (including new test).
• uv run ruff check autogen/agents/experimental/a2ui/schema_manager.py test/agents/experimental/a2ui/test_schema_manager.py — clean.
• uv run ruff format --check ... — already formatted.

Note: the a2ui test module is gated on the a2a optional dependency (a2a-sdk>=1.0.0,<2); installed locally via uv pip install before running the suite.

AI assistance

Diff drafted with assistance and reviewed end-to-end against the existing UTF-8 pin convention used in the surrounding codebase. The regression test exercises the catalog file-load branch with mixed-script bytes through both the $id and a nested component description, mirroring the read-side tests added in #2909 / #2910.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
autogen/agents/experimental/a2ui/schema_manager.py	82.19% <100.00%> (+82.19%)	⬆️

... and 535 files with indirect coverage changes

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