more events for paras pallet

[Lynette7]

Adds events to the paras pallet to enable external parties (like exchanges, indexers, and infrastructure providers) to monitor parachain runtime upgrades without constantly polling storage.

Closes #10074

Problem

Currently, when a parachain schedules a runtime upgrade, no event is emitted for external parties to listen to. This makes it difficult for exchanges and other infrastructure providers to:

• Know when to expect transaction invalidations
• Prepare their systems for parachain upgrades
• Monitor the upgrade lifecycle

Changes

New Events Added:

• CodeUpgradeScheduledAt(ParaId, BlockNumber) - Emitted when a code upgrade is scheduled
• UpgradeGoAheadSignalSet(ParaId) - Emitted when the go-ahead signal is set
• UpgradeRestrictionSignalSet(ParaId) - Emitted when upgrade restrictions are applied
• UpgradeRestrictionSignalRemoved(ParaId) - Emitted when upgrade restrictions are removed
• FutureCodeUpgradeRemoved(ParaId) - Emitted when a future upgrade is cancelled or applied

Event Emission Points:

• proceed_with_upgrade() - Emits CodeUpgradeScheduledAt
• process_scheduled_upgrade_changes() - Emits UpgradeGoAheadSignalSet
• process_scheduled_upgrade_cooldowns() - Emits UpgradeRestrictionSignalRemoved
• note_new_head() - Emits FutureCodeUpgradeRemoved
• schedule_code_upgrade() - Emits UpgradeRestrictionSignalSet
• enact_pvf_rejected() - Emits UpgradeGoAheadSignalSet and FutureCodeUpgradeRemoved

Testing

Added comprehensive tests covering:

• Event emission for all upgrade strategies
• Event emission during PVF rejection
• Event emission for multiple parachains
• Event emission throughout the upgrade lifecycle

Performance Impact

Events have minimal performance impact:

• Only simple data (ParaId, BlockNumber) is emitted
• No additional storage reads/writes beyond existing logic
• Events are emitted in already-benchmarked code paths

Checklist

• Added new events to track parachain upgrades
• Added comprehensive tests for all new events
• Events follow existing naming conventions

[cla-bot-2021]

User @Lynette7, please sign the CLA here.

[Lynette7]

@bkchr could I get feedback on this PR? Do you think the changes proposed here are valid and/or necessary? This is my first PR to the Polkadot SDK and I'm looking to learn so any feedback is highly appreciated.