fix: follow-up correctness fixes for #230 and #234

[Yifan Yang (Yif-Yang)]

Follow-up to #230 and #234, both merged. Each shipped a defect that made the
feature not actually work in the case it was written for.

1. harvest_opencode: the Roaming branch was unreachable (#230)

LOCALAPPDATA or APPDATA resolves to the former in virtually every Windows
session, so the %APPDATA% half of the new branch was dead code and a database
that really lives under Roaming was never found.

Resolve the data root from ordered candidates instead and pick the one that
actually holds opencode.db — the same shape as _devin_transcript_candidates

• _first_existing in plugins/devin/harvest_devin.py. When no database exists
  yet the Local root stays the reported default, so error messages are unchanged.
  A relative OPENCODE_DB now resolves below the same selected root.

Repro before this patch (both env vars set, db under Roaming) — always picked Local:

sys.platform = "win32"
os.environ["LOCALAPPDATA"] = r"C:\Users\u\AppData\Local"
os.environ["APPDATA"]      = r"C:\Users\u\AppData\Roaming"   # db actually lives here
default_opencode_db()  # -> ...\AppData\Local\opencode\opencode.db

2. minimax_backend: region selection clobbered an explicit base URL (#234)

configure_minimax_chat applied the region default unconditionally and wrote it
into os.environ["MINIMAX_BASE_URL"]. Since trainer.py and eval_only.py pass
cfg.get("minimax_base_url") or None — i.e. None whenever the user did not set
it in YAML — configuring model.minimax_region silently discarded a proxy or
private gateway that the user had set through the environment.

Repro before this patch:

import-time BASE_URL: https://my-corp-proxy.internal/v1
after configure(region="cn_zh"): https://api.minimaxi.com/v1   # proxy gone

Track whether the current base URL was chosen explicitly and only fill in the
region default when it was not. This makes the code match what the docs already
state (docs/guide/configuration.md: "an explicit model.minimax_base_url or
MINIMAX_BASE_URL overrides the region default") — previously the docs were
right and the code was wrong. Region selection still works normally when the
base URL is blank or unset.

Tests

Added regression tests covering exactly the scenarios above:

• test_default_database_honors_windows_roaming_appdata — APPDATA-only session
• test_default_database_prefers_the_appdata_root_that_has_the_database — both set, db under Roaming
• test_default_database_prefers_local_appdata_when_neither_exists — messaging fallback
• test_relative_opencode_db_resolves_below_the_selected_windows_root
• test_env_base_url_survives_a_later_region_selection
• test_explicit_base_url_survives_a_later_region_only_call
• test_region_still_applies_when_base_url_is_blank

Full suite on this branch: 1114 passed, 10 skipped, 130 subtests passed.

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