fix(dotnet): consume all MSBuild BuildEventArgsFieldFlags in binlog reader - #4620

Summary

Consume all MSBuild BuildEventArgsFieldFlags in read_event_fields in MSBuild's serialization order so structured RECORD_WARNING and RECORD_ERROR events preserve their diagnostic code, file path, line, and column.

Closes #4587

Context & Problem

In MSBuild's binary log format (BuildEventArgsWriter.WriteBaseFields), base event fields are written in this order when their BuildEventArgsFieldFlags bits are set:

1. Message (1 << 2): deduplicated string
2. BuildEventContext (1 << 0): 7 x 7-bit ints (6 in v1)
3. ThreadId (1 << 4): 7-bit int
4. HelpKeyword (1 << 1): deduplicated string
5. SenderName (1 << 3): deduplicated string (set whenever SenderName != null && SenderName != "MSBuild", such as "Csc" for compiler warnings and errors)
6. Timestamp (1 << 5): i64 ticks (8 bytes LE) + DateTimeKind (7-bit int)
7. Extended (1 << 16): extended type, dictionary, and extended comment

Followed by optional message diagnostic/location flags (Subcategory, Code, File, ProjectFile, LineNumber, ColumnNumber, EndLineNumber, EndColumnNumber), Arguments, and Importance.

src/cmds/dotnet/binlog.rs only handled Message, BuildEventContext, Timestamp, Extended, Arguments, and Importance. When a task such as Csc logged a warning or error with SenderName = "Csc", read_event_fields did not read the 1-byte SenderName string index before Timestamp. As a result, Timestamp's read_i64_le() + read_7bit_i32() consumed SenderName + the 8-byte Ticks (whose high byte is < 0x80), leaving Timestamp.Kind unread in the stream.

Subsequent reads in RECORD_ERROR | RECORD_WARNING were shifted by one field:

• _subcategory read Timestamp.Kind
• code read Subcategory ("")
• file read Code (for example "CS0168")
• _project_file read File (for example "Beta.cs")
• line read the ProjectFile string index (for example 53)
• column read LineNumber (for example 7)

This produced corrupted warning output such as CS0168(53,7): The variable 'unused' is declared... with the source file path lost.

Solution

• Define the remaining FLAG_* constants from MSBuild's BuildEventArgsFieldFlags in src/cmds/dotnet/binlog.rs.
• Read ThreadId, HelpKeyword, SenderName, and optional message diagnostic/location fields in read_event_fields in MSBuild's exact serialization order.
• Add a unit test (test_parse_build_reads_events_with_sender_name_and_optional_flags) verifying that RECORD_WARNING and RECORD_ERROR events with BuildEventContext, ThreadId, HelpKeyword, SenderName, and Timestamp retain their code, file, line, column, and message.

How to Test

Verified by static code inspection against MSBuild's BuildEventArgsFieldFlags / BuildEventArgsWriter / BuildEventArgsReader and src/cmds/dotnet/binlog.rs; execution is delegated to the PR CI matrix:

cargo test --lib cmds::dotnet::binlog::tests

Checklist

• cargo test passes (pending CI execution; not run locally)
• cargo clippy --all-targets clean (pending CI execution; not run locally)
• cargo fmt --check clean (pending CI execution; not run locally)
• Single-line format adds [\u{26a1} N tokens] badge (via format_badge()) (N/A: binlog parser bug fix)
• Multi-line format ends with summary_footer() line (N/A: binlog parser bug fix)
• Saved tokens > 0 on realistic fixture (no negative savings) (N/A: binlog parser bug fix)