build: pin ethnum to 1.5.3

[0xkaushik-ai]

Description of Changes

Pins the workspace ethnum dependency to exactly 1.5.3. Versions before 1.5.3 rely on the representation of an error type that changed in newer Rust toolchains, which can break downstream users even though SpacetimeDB's pinned toolchain is unaffected.

The lockfile already resolved ethnum 1.5.3, so no lockfile update is required.

Fixes #5512.

API and ABI breaking changes

None.

Expected complexity level and risk

1. This is a one-line workspace dependency constraint change to the version already present in Cargo.lock.

Testing

• cargo metadata --locked --no-deps --format-version 1
• cargo check --locked -p spacetimedb-sats -p spacetimedb-expr -p spacetimedb-table -p spacetimedb-memory-usage
• cargo test --locked -p spacetimedb-sats -p spacetimedb-expr -p spacetimedb-table -p spacetimedb-memory-usage --lib
• git diff --check

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1 out of 2 committers have signed the CLA.

✅ bfops
❌ 0xkaushik-ai
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[bfops]

Thanks for the PR. Have you confirmed that this resolves the issue that would previously break downstream users?

[bfops]

I was unable to repro the original issue without this PR (presumably because cargo chose the latest compatible version anyway) but still reasonable to bump the version. Added a comment and remove the = constraint since default constraint is good enough.

[bfops]

I haven't heard back from @0xkaushik-ai and the CLA is unsigned, so I'm going to close this PR. Feel free to reopen if you'd like to sign the CLA and get this merged!