autogen-studio: pin utf-8 encoding on production text-file open() calls (refs #5566)

[adv0r]

Dear maintainer — this PR has a permanent home with methodology + opt-out at tokens-for-good. A one-line "no thanks" → auto-close + blacklist. Sorry for the notification this edit caused.

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Why

Refs #5566. Continuation of the encoding sweep started in #6094
(playwright_controller.py) and continued in
the magentic-one-cli PR.

The original report explicitly flagged that "there will be some
similar issues in the codebase while using open function". On a
non-UTF-8 default locale (cp950 on Traditional Chinese Windows, cp1252
on Western European Windows, …), Python's open(..., \"r\") falls back
to the platform encoding and crashes with UnicodeDecodeError on any
non-ASCII byte.

This PR closes the autogen-studio production code paths that read
or write text files without an explicit encoding.

What changed

5 files, 11 open() call sites, all of the same shape:

- with open(path, \"r\") as f:
+ with open(path, \"r\", encoding=\"utf-8\") as f:

File	sites
autogenstudio/cli.py	1 (writes runtime .env)
autogenstudio/lite/studio.py	1 (writes runtime .env)
autogenstudio/database/schema_manager.py	6 (Alembic env.py / script.py.mako / alembic.ini)
autogenstudio/web/auth/manager.py	1 (user-supplied YAML config)
autogenstudio/gallery/builder.py	1 (writes gallery_default.json)

Why these specific sites

• Alembic templates and env.py can legitimately contain non-ASCII
  comments / paths and are read+rewritten on schema upgrades.
• .env writers are called with the user's project path. Folder
  names with accented characters (very common on Windows) would crash
  the first run.
• YAML auth config is user-supplied.
• gallery_default.json can contain non-ASCII strings.

Scope deliberately narrowed

• Production-code only. Test fixtures left out for a separate PR.
• Skipped aiofiles.open in teammanager.py — async API signature
  is slightly different, deserves its own audited PR.
• Did NOT sweep agbench/benchmarks/* — those are scenario scripts
  that consume JSONL produced by other agents; forcing UTF-8 there
  could mask issues upstream.

Verification

• ast.parse(...) clean on all 5 touched files (no syntax break).
• No encoding=...encoding= (double-add) anywhere.
• 11 insertions, 11 deletions (same-line edits only).
• No behaviour change for users already on UTF-8 locales.

Recommended next sweep (for a separate PR)

• agbench (mixed: some files read agent-emitted JSONL, others are
  user scripts — needs case-by-case audit)
• async aiofiles.open sites

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[adv0r]

Heads up: the CLA reply needs to come from the human account holder (@adv0r) directly, which I can't auto-post on their behalf in good conscience — the magic-phrase reply is a binding legal acceptance. I've flagged it on the user's side as a manual TODO and the CLA acceptance should land here shortly. The companion PR #7722 already shows license/cla: SUCCESS, so this looks like a per-PR re-acknowledgment after the initial sign.

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