feat: add REG_GB (London Stock Exchange) core region support

[kaish114]

Summary

This PR adds core framework support for REG_GB (London Stock Exchange) as a new trading region in Qlib, complementing the Yahoo Finance data collector support added in #2155.

Changes:

• qlib/constant.py: Add REG_GB = "gb" constant
• qlib/config.py: Add REG_GB to _default_region_config with LSE-appropriate defaults (trade_unit=1, limit_threshold=None, deal_price="close")
• qlib/utils/time.py: Add GB_TIME (08:00–16:30, no midday break) and wire REG_GB into get_min_cal, is_single_value, and time_to_day_index
• tests/misc/test_utils.py: Add GBTimeUtils test class covering get_min_cal, is_single_value, and time_to_day_index for REG_GB

LSE trading hours: 08:00–16:30 Europe/London, continuous session (no lunch break), 510 one-minute bars per day.

Timezone & DST note

GB_TIME uses timezone-naive local hours (08:00–16:30), consistent with how all other regions (including REG_US which also observes DST) are handled in Qlib. The LSE always opens and closes at the same local time regardless of GMT/BST, so the 510-bar calendar is stable year-round. DST resolution for minute-level data is handled at collection time via _timezone = "Europe/London" in the Yahoo Finance collector (PR #2155). This is the same design assumption as the existing US region implementation.

Test plan

• tests/misc/test_utils.py::TimeUtils::test_cal_sam_minute passes with REG_GB in the regions list
• tests/misc/test_utils.py::GBTimeUtils — 10 new tests: calendar count (510 bars), open/close times, is_single_value (last bar, mid-session, sub-freq window), time_to_day_index (open, mid, last bar, out-of-range ValueError)
• All 12 tests in tests/misc/test_utils.py pass
• black -l 120 --target-version py311 — no changes needed

[kaish114]

CI failures are pre-existing infrastructure issues, not related to this PR

All failing checks trace back to two pre-existing issues in the CI environment — neither is caused by changes in this PR.

1. test_pit.py — East Money API unreachable from GitHub Actions runners

The test_pit.py setup calls get_calendar_list() in scripts/data_collector/utils.py, which makes an HTTP request to the East Money (东方财富) API to fetch the CSI300 trading calendar:

GET https://push2his.eastmoney.com/api/qt/stock/kline/get?secid=1.000300&...
ConnectionError: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response'))

This API is unreachable from GitHub Actions runners (geo-restricted or rate-limited outside China). All 5 test_pit.py tests pass locally.

2. rl/test_trainer.py::test_trainer_earlystop — flaky float comparison

FAILED rl/test_trainer.py::test_trainer_earlystop - assert 30.0 > 30

Pre-existing floating-point precision issue in the RL trainer test, unrelated to this PR.

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This PR's tests — all pass

This PR only modifies qlib/constant.py, qlib/config.py, qlib/utils/time.py, and tests/misc/test_utils.py. Running the full test suite locally (Python 3.13, macOS):

misc/test_utils.py ............   [12/12 passed]

All 12 tests in misc/test_utils.py pass, including the 10 new GBTimeUtils tests added in this PR.