Require precompiled Rust modules in smoketest construction

[pederbe]

Description of Changes

Use precompiled fixtures for ordinary Rust smoketests, including the default noop module and column-default migrations. The HTTP tutorial fixture compiles Rust blocks directly from the documentation during warmup.

Remove the source-string constructor and runtime Rust publish fallback. Build-diagnostic tests use a separate helper that invokes spacetime build without starting a server. The namespace-collision test now requires the specific compilation diagnostic instead of accepting any publish failure.

Fixes #5653.

API and ABI breaking changes

Removes source-based helpers from the internal smoketest harness. Product APIs and ABIs are unaffected.

Rollback safety impact

n/a

Expected complexity level and risk

2/5. The changes affect test setup, fixture compilation, and build-diagnostic coverage.

Testing

Validated on Linux with Rust 1.93.0 and freshly built CLI/standalone binaries:

• Built the complete precompiled module workspace.
• Passed 3 harness unit tests, 5 focused module/default/tutorial tests, and 3 build-diagnostic tests.
• Broader migration, view, SQL, and HTTP run: 47 passed; 5 could not run because .NET or pnpm was unavailable.
• Passed all-target smoketest Clippy with warnings denied and formatting checks for both workspaces.
• Verified tutorial extraction with LF/CRLF, missing and unterminated code blocks, rebuild after documentation edits, and compilation failure for invalid tutorial Rust. Restored the document and verified the original WASM output.

[CLAassistant]

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