feat: upgrade subxt to 0.39.0

[nmammeri]

Pull Request

Closes: PRO-2026

Checklist

Please conduct a thorough self-review before opening the PR.

• I am confident that the code works.
• I have written sufficient tests.
• I have written and tested required migrations.
• I have updated documentation where appropriate.

Summary

• Subxt 0.39.0 introduces breaking changes. subxt is now completely decoupled from subtrate and doesn't require it as dependency (substrate-compat feature is removed)
• They have introduced basic types instead of substrate types such as: subxt::utils::H256 instead of sp_core::H256.
• The changes made in this PR mainly address this type conversion issues especially for the dot chain.
• There were a lot of assumptions that the dot chain hash type is sp_core::H256, this PR changes the dot hash type to be pub type PolkadotHash = <subxt::PolkadotConfig as subxt::Config>::Hash;
• PR passes full bouncer tests

[codecov]

Codecov Report

Attention: Patch coverage is 1.42857% with 69 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
engine/src/lib.rs	0%	28 Missing ⚠️
engine/src/dot/http_rpc.rs	0%	22 Missing ⚠️
engine/src/witness/dot.rs	0%	14 Missing ⚠️
engine/src/state_chain_observer/client.rs	0%	3 Missing ⚠️
engine/src/dot/retry_rpc.rs	50%	1 Missing ⚠️
engine/src/dot/rpc.rs	0%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[dandanlen]

Please don't merge yet - I want to look into this to see if we can avoid importing polkadot-sdk a second time 🙈