fix: disable sparse output optimization when AST is requested

[gakonst]

Summary

When AST output is requested via --ast flag, sparse output optimization must be disabled because AST node IDs are specific to each compilation unit.

Problem

When compiling with forge test --ast, if a test file imports a source file that hasn't changed (cache hit), the sparse output optimization would request empty output selection for that cached import. This causes:

1. The test file gets freshly compiled with AST containing references to the imported contract
2. The imported contract's cached AST has different node IDs (from a previous compilation with different sources)
3. Cross-file AST references are broken because node IDs don't match

Fix

This PR adds:

• OutputSelection::contains_ast() method to detect if AST is in the output selection
• Logic in sparse_sources to skip sparse optimization when AST is requested

When AST is detected in the output selection, all files in the compilation unit get full output selection, ensuring consistent AST node IDs across all files.

Testing

Added unit test for contains_ast() covering:

• Explicit AST selection (ast_output_selection())
• Complete output selection (includes * wildcard)
• Default output selection (no AST)
• Empty output selection
• Contract-only outputs

Fixes foundry-rs/foundry#11056

Reproduction

# Clone reproduction repo
git clone https://github.com/ferranbt/test-ast-update
cd test-ast-update

# Clear cache and compile first test
rm -rf out cache
forge test --match-test test_Increment --match-path test/Counter.t.sol --ast

# Compile second test
forge test --match-test test_Other --match-path test/Other.t.sol --ast

# Compare node IDs - they should match but don't without this fix
cat out/Other.sol/Other.json | jq ".ast.exportedSymbols.Other.[0]"
cat out/Other.t.sol/OtherTest.json | jq ".ast.exportedSymbols.Other.[0]"

[onbjerg]

rationale & fix lgtm