feat(explorer): Add Code Mode toggle to Explorer UI

[azulus]

Summary

• Adds a CM (Code Mode) toggle to the Explorer top bar, next to the existing CE toggle
• Only visible when user has seer-explorer-code-mode-tools feature flag
• Frontend sends override_code_mode_enable in POST body
• Backend uses it to enable/disable code mode tools (only overrides if flag is already enabled — can't bypass the flag)

Test plan

• Verify toggle appears only when feature-flagged
• Verify toggling CM off suppresses code mode tools in Explorer responses
• Verify CE toggle still works independently

🤖 Generated with Claude Code

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[github-actions]

Backend Test Failures

Failures on 03d7cf9 in this run:

tests/snuba/api/endpoints/test_organization_events_timeseries_spans.py::OrganizationEventsStatsSpansMetricsEndpointTest::test_handle_nans_from_snuba_top_n — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/snuba/api/endpoints/test_organization_events_timeseries_spans.py:212: in test_handle_nans_from_snuba_top_n
    assert timeseries["values"] == build_expected_timeseries(
E   AssertionError: assert [{'confidence...ne, ...}, ...] == [{'confidence...Y>, ...}, ...]
E     
E     At index 6 diff: {'timestamp': 1776038400000, 'value': 2.0, 'incomplete': True, 'incompleteReason': 'INCOMPLETE_BUCKET', 'sampleCount': 1, 'sampleRate': 1.0, 'confidence': 'low'} != {'incomplete': False, 'timestamp': 1776038400000.0, 'value': 2, 'sampleCount': <ANY>, 'sampleRate': <ANY>, 'confidence': <ANY>}
E     
E     Full diff:
E       [
E           {
E     -         'confidence': <ANY>,
E     ?                       -- ^^
E     +         'confidence': None,
E     ?                        ^^^
E               'incomplete': False,
E     -         'sampleCount': <ANY>,
E     ?                        ^^^^^
E     +         'sampleCount': 0,
E     ?                        ^
E     -         'sampleRate': <ANY>,
E     ?                       -- ^^
E     +         'sampleRate': None,
E     ?                        ^^^
E     -         'timestamp': 1775520000000.0,
E     ?                                   --
E     +         'timestamp': 1775520000000,
E     -         'value': 0,
E     +         'value': 0.0,
E     ?                  ++
E           },
E           {
E     -         'confidence': <ANY>,
E     ?                       -- ^^
E     +         'confidence': None,
E     ?                        ^^^
E               'incomplete': False,
E     -         'sampleCount': <ANY>,
E     ?                        ^^^^^
E     +         'sampleCount': 0,
E     ?                        ^
E     -         'sampleRate': <ANY>,
E     ?                       -- ^^
E     +         'sampleRate': None,
E     ?                        ^^^
E     -         'timestamp': 1775606400000.0,
E     ?                                   --
E     +         'timestamp': 1775606400000,
E     -         'value': 0,
E     +         'value': 0.0,
E     ?                  ++
... (111 more lines)