[markdown] Simplify deindentation logic for fenced code block lines

[parlough]

Fenced code blocks track leading spaces before the opening code fence to remove indentation from each line in the code block.

The pre-existing function for doing this removed the correct amount of indentation but then recreated the same string. We could simplify the function by justing removing the second substring. However, since this is called for every line of each code block and it's only necessary to handle spaces, I instead rewrote the function to remove the regular expression usage.

[gemini-code-assist]

Code Review

This pull request simplifies and optimizes the de-indentation logic for fenced code block lines by replacing a regular expression-based implementation with a more direct, character-by-character check. This change improves performance and also more strictly adheres to the CommonMark specification by only considering spaces for indentation removal, not all whitespace characters. The new implementation is correct and a clear improvement. I have one minor suggestion to improve readability.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:bazel_worker	1.1.3	already published at pub.dev
package:benchmark_harness	2.4.0-wip	WIP (no publish necessary)
package:boolean_selector	2.1.2	already published at pub.dev
package:browser_launcher	1.1.3	already published at pub.dev
package:cli_config	0.2.1-wip	WIP (no publish necessary)
package:cli_util	0.5.0-wip	WIP (no publish necessary)
package:clock	1.1.3-wip	WIP (no publish necessary)
package:code_builder	4.11.0	already published at pub.dev
package:coverage	1.15.0	already published at pub.dev
package:csslib	1.0.2	already published at pub.dev
package:extension_discovery	2.1.0	already published at pub.dev
package:file	7.0.2-wip	WIP (no publish necessary)
package:file_testing	3.1.0-wip	WIP (no publish necessary)
package:glob	2.1.3	already published at pub.dev
package:graphs	2.3.3-wip	WIP (no publish necessary)
package:html	0.15.6	already published at pub.dev
package:io	1.1.0-wip	WIP (no publish necessary)
package:json_rpc_2	4.0.0	already published at pub.dev
package:markdown	7.3.1-wip	WIP (no publish necessary)
package:mime	2.0.0	already published at pub.dev
package:oauth2	2.0.4-wip	WIP (no publish necessary)
package:package_config	2.3.0-wip	WIP (no publish necessary)
package:pool	1.5.2	already published at pub.dev
package:process	5.0.5	already published at pub.dev
package:pub_semver	2.2.0	already published at pub.dev
package:pubspec_parse	1.5.0	already published at pub.dev
package:source_map_stack_trace	2.1.3-wip	WIP (no publish necessary)
package:source_maps	0.10.14-wip	WIP (no publish necessary)
package:source_span	1.10.1	already published at pub.dev
package:sse	4.1.8	already published at pub.dev
package:stack_trace	1.12.1	already published at pub.dev
package:stream_channel	2.1.4	already published at pub.dev
package:stream_transform	2.1.2-wip	WIP (no publish necessary)
package:string_scanner	1.4.1	already published at pub.dev
package:term_glyph	1.2.3-wip	WIP (no publish necessary)
package:test_reflective_loader	0.4.0	already published at pub.dev
package:timing	1.0.2	already published at pub.dev
package:unified_analytics	8.0.5	already published at pub.dev
package:watcher	1.1.4	ready to publish	watcher-v1.1.4
package:yaml	3.1.3	already published at pub.dev
package:yaml_edit	2.2.2	already published at pub.dev

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[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
markdown	None	7.3.0	7.3.1-wip	7.3.0	✔️

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Coverage ✔️

File	Coverage
pkgs/markdown/lib/src/block_syntaxes/fenced_code_block_syntax.dart	💚 98 % ⬆️ 0 %

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
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pkgs/bazel_worker/benchmark/benchmark.dart
pkgs/bazel_worker/example/client.dart
pkgs/bazel_worker/example/worker.dart
pkgs/benchmark_harness/integration_test/perf_benchmark_test.dart
pkgs/boolean_selector/example/example.dart
pkgs/clock/lib/clock.dart
pkgs/clock/lib/src/clock.dart
pkgs/clock/lib/src/default.dart
pkgs/clock/lib/src/stopwatch.dart
pkgs/clock/lib/src/utils.dart
pkgs/clock/test/clock_test.dart
pkgs/clock/test/default_test.dart
pkgs/clock/test/stopwatch_test.dart
pkgs/clock/test/utils.dart
pkgs/coverage/lib/src/coverage_options.dart
pkgs/html/example/main.dart
pkgs/html/lib/dom.dart
pkgs/html/lib/dom_parsing.dart
pkgs/html/lib/html_escape.dart
pkgs/html/lib/parser.dart
pkgs/html/lib/src/constants.dart
pkgs/html/lib/src/encoding_parser.dart
pkgs/html/lib/src/html_input_stream.dart
pkgs/html/lib/src/list_proxy.dart
pkgs/html/lib/src/query_selector.dart
pkgs/html/lib/src/token.dart
pkgs/html/lib/src/tokenizer.dart
pkgs/html/lib/src/treebuilder.dart
pkgs/html/lib/src/utils.dart
pkgs/html/test/dom_test.dart
pkgs/html/test/parser_feature_test.dart
pkgs/html/test/parser_test.dart
pkgs/html/test/query_selector_test.dart
pkgs/html/test/selectors/level1_baseline_test.dart
pkgs/html/test/selectors/level1_lib.dart
pkgs/html/test/selectors/selectors.dart
pkgs/html/test/support.dart
pkgs/html/test/tokenizer_test.dart
pkgs/html/test/trie_test.dart
pkgs/html/tool/generate_trie.dart
pkgs/pubspec_parse/test/git_uri_test.dart
pkgs/stack_trace/example/example.dart
pkgs/watcher/test/custom_watcher_factory_test.dart
pkgs/yaml_edit/example/example.dart

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[srawlins]

Lovely!