pallet-bounties: extends update_due infinitly if BountiesUpdatePeriod…

[BigTava]

… is None

Description

Fixes polkadot-fellows/runtimes#509

The Bounties expiration and renewal heavily depends on manual interactions through UI. This PR refactors the duration of the bounty to be an optional configuration constant. If set to None, bounties remain active indefinitely, removing the need for callingextend_bounty_expiry and preventing automatic curator slashing for inactivity, which often penalises unnecessarily.

Integration

Remove BountyUpdatePeriod

Review Notes

Modifies how bounty expiry is handled

🔍 Code Diff Summary

- #[pallet::constant]
- type BountyUpdatePeriod: Get<BlockNumberFor<Self, I>>;
+ #[pallet::constant]
+ type BountyUpdatePeriod: Get<Option<BlockNumberFor<Self, I>>>;

- *update_due = (Self::treasury_block_number() + T::BountyUpdatePeriod::get()).max(*update_due);
+ *update_due = Self::treasury_block_number().saturating_add(
+     T::BountyUpdatePeriod::get().unwrap_or(BlockNumberFor::<T, I>::MAX)
+ ); 

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
• My PR follows the labeling requirements of this project (at minimum one label for T required)
  • External contributors: ask maintainers to put the right label on your PR.
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[muharem]

this will be included in the release notes. I would explain what this means. this is more like a description of the implementation, it can be read from the code.

also please add PR description

[BigTava]

Updated. Thanks!

[muharem]

it was a poor doc for this parameter, lets give more context. explain behaviour.
".. it is considered BlockNumber::MAX" is description of the implementation. what it means for a user.

[BigTava]

Updated. Thanks!

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/13568501540
Failed job name: test-linux-stable-no-try-runtime

[muharem]

Suggested change

	If slashing of curators is necessary, it can be handled through OpenGov.
	This reduces friction in curator management.

this is a bit specific to Polkadot, but the rest looks good

[muharem]

Suggested change

	/// Optional bounty duration in blocks.
	/// The period that starts when a curator is approved, during which they must execute or
	/// update the bounty via `extend_bounty_expiry`. If missed, the bounty expires, and the
	/// curator may be slashed.

[muharem]

Suggested change

	/// If `None`, bounties stay active indefinitely, removing the need for
	/// `extend_bounty_expiry` and restricting curator slashing to OpenGov.
	/// If `None`, bounties stay active indefinitely, removing the need for
	/// `extend_bounty_expiry`.

OpenGov part is specific to the runtime e