fix(axle): boost the import rate during an Axle export event

[springfall2008]

Summary

• An Axle export event pays a premium to export power, so charging (importing) instead during the same window carries the same opportunity cost. load_axle_slot only ever boosted rate_export for these events, leaving rate_import completely untouched — Predbat could plan to charge cheaply through an export event just because the normal import rate happened to be low at that time, with no cost reflected in the final plan/figure.
• load_axle_slot now applies an export-direction session's pence_per_kwh to both rate_export and rate_import, mirroring how Octopus saving sessions already boost both directions for the same event.
• Managed-mode's price curve conversion (_process_price_curve) no longer needs to fabricate a second, sign-negated "import" session per half-hourly slot to get the same net effect — that trick is removed, since a single export-direction session now covers both sides of the meter. Net rate impact for managed mode is unchanged.
• Added a doc note under Axle VPP in docs/energy-rates.md explaining the boost applies to both rates during an event.

Test plan

• New unit test _test_axle_load_slot_export_boosts_import_rate — verifies load_axle_slot raises rate_import for an export-direction session.
• New unit test _test_axle_byok_export_event_boosts_import_rate — end-to-end via fetch_axle_sessions + load_axle_slot, confirms a single BYOK export event boosts both rate_import and rate_export with no synthetic duplicate session needed.
• Updated managed-mode tests (managed_price_curve, managed_fetch_e2e, managed_token_retry) for the new single-session-per-slot price curve output.
• Full axle test suite: 32/32 passed.
• ./run_all --quick: all tests passed, no regressions.
• ./run_pre_commit: all hooks passed (ruff, black, cspell, etc).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates Axle VPP event handling so that export-direction events increase both rate_export and rate_import, ensuring Predbat models the opportunity cost of charging during premium export windows. It also simplifies managed-mode price curve processing by removing the need to generate synthetic “import” sessions per slot, since a single export-direction session now impacts both directions.

Changes:

• Apply export-session pence_per_kwh to both import and export rates in load_axle_slot.
• Simplify managed-mode _process_price_curve to emit one export-direction session per slot (no negated import companion).
• Add/adjust Axle unit tests and document the new “boost both rates” behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/energy-rates.md	Documents that Axle export events boost both import and export rates to reflect opportunity cost.
apps/predbat/axle.py	Updates load_axle_slot export-session handling to raise both rates; simplifies managed price-curve session generation.
apps/predbat/tests/test_axle.py	Adds regression tests for import-rate boosting during export events and updates managed-mode expectations for single-session-per-slot.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (1)

apps/predbat/axle.py:668

• axle_sessions can include up to 7 days of history (see cleanup_event_history), so minutes_to_time() can yield negative start_minutes/end_minutes for sessions that started before midnight_utc. Now that import_export == "export" sessions also mutate rate_import (export=False call), the current loop can create negative minute keys in rate_dict/rate_replicate (because it uses rate_dict.get(minute, 0)), which can bloat these dicts and make debugging/replication data confusing.

Clamp the window to non-negative minutes (and skip empty windows) before iterating.

        if start_minutes is not None and end_minutes is not None and start_minutes < (base.forecast_minutes + base.minutes_now):
            if import_export == "export":
                # An export event pays a premium to export, so charging instead during the same
                # window carries the same opportunity cost - apply the same boost to both the
                # export and the import rate (mirrors how Octopus saving sessions boost both