fix(chain)!: handle descriptor SPK collisions

[busayo-OD]

Description

Fixes #2277.

KeychainTxOutIndex does not currently handle collisions when a descriptor derives an SPK that is already owned by another keychain.

This PR allows wildcard descriptors to skip already-owned SPKs when revealing new scripts and continue to the next available SPK. Non-wildcard descriptors are rejected when their only SPK is already claimed, returning the new NoDerivableSpk error variant.

Notes to the reviewers

• The rejected non-wildcard descriptor is not registered and its state is rolled back.
• Collided SPKs remain associated with their original keychains and are not recorded as revealed for the wildcard keychain.
• Regression tests cover non-wildcard descriptor rejection, both insertion orders, and consecutive SPK collisions.
• The new NoDerivableSpk error needs to be handled in bdk_wallet by mapping it to DescriptorOutputCollision.

Changelog notice

• Add NoDerivableSpk to InsertDescriptorError.
• Handle SPK collisions when revealing wildcard descriptor outputs.
• Reject non-wildcard descriptors whose only SPK is already claimed.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before committing

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR