fix: update fedimint to resolve iroh response limit

[bansalayush247]

Summary

Update the Fedimint dependencies from v0.10.0 to v0.12.0 to address the Iroh response-size limitation affecting federation synchronization.

Background

The issue was investigated and the 1 MB Iroh response limit in Fedimint v0.10.0 was identified as the strongest candidate.

In v0.10.0, Iroh responses are limited to 1,000,000 bytes. A single completed federation session can exceed this limit, causing the Observer's history worker to fail while fetching that session.

Since the Observer processes sessions in order, a permanently failing session can prevent subsequent sessions from reaching database processing, effectively stopping synchronization for that federation.

The upstream fix for this response limit is included in Fedimint v0.11.0 and later. This PR updates the Observer's Fedimint dependencies to v0.12.0.

Changes

• Update all workspace Fedimint dependencies from v0.10.0 to v0.12.0
• Update Cargo.lock to reflect the new dependency resolution

Expected Behavior

Federation synchronization should no longer fail due to the previous Iroh response-size limitation, allowing the Observer to continue processing sessions beyond the affected session.

Investigation

This issue was investigated by @elsirion. The detailed investigation is included below:

Thanks @elsirion for the detailed investigation and diagnosis.

Testing

• cargo check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test

Close #125