cmd/cover: exclude commented-out code from coverage instrumentation

[rudy-regazzoni-sonarsource]

Add logic to exclude purely commented lines from coverage instrumentation.

When instrumenting Go code for coverage, the cover tool now identifies
and excludes lines that contain only comments from coverage blocks.
This prevents commented-out code from being reported as "uncovered"
in coverage reports, which can be misleading.

The implementation adds a splitBlockByComments function that parses
source code character by character to identify segments containing
executable code versus segments containing only comments. The
addCounters function now uses this to create coverage counters only
for segments that contain actual executable code.

The parser correctly handles:

• Single-line comments (//)
• Multi-line comments (/* */)
• String literals containing comment-like sequences
• Raw string literals with fake comments
• Mixed lines with both code and comments

This improves the accuracy of coverage reports by ensuring that
commented-out code, TODOs, and documentation comments don't inflate
the count of uncovered lines.

Fixes #22545

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Patch Set 1:

(1 comment)

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Patch Set 2:

(1 comment)

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(5 comments)

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