[mustache_template] Adopt code-excerpts for README

[DaNiELChIoRo]

Replaces the hand-written Dart snippets in mustache_template's README with <?code-excerpt?> pragmas backed by a new example/lib/readme_excerpts.dart, so every snippet is validated against compilable, analyzed source instead of free-hand text (dart pub global run flutter_plugin_tools update-excerpts / validate both pass clean).

This surfaced two pre-existing doc bugs:

• Template('{{# foo }}') in the first lambda example had no closing tag, so it threw a parse error the moment it was actually executed.
• The two <b>PUB</b> lambda examples were an exact copy-pasted duplicate; the duplicate is removed.

Also removes ci_config.yaml, whose only purpose was opting the package out of excerpt validation (see #102679-style TODO in the file) — no longer needed now that the README is excerpt-backed.

Note on overlap: flutter/flutter#183936 already has two other open PRs addressing it — #23 and #36. This PR was written independently and additionally removes the ci_config.yaml exemption, which neither of those does. Happy to have maintainers close this in favor of one of those if that's preferred; flagging here so reviewer time isn't wasted on duplicate review.

Fixes flutter/flutter#183936

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the Flutter style guide and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [vector_math]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

This PR only touches the README, CHANGELOG, an example app, and a CI opt-out file — no package source changed, so it falls under the documentation/example test exemption. The new example/lib/readme_excerpts.dart is exercised directly (dart run) as part of verifying this change, and every existing test in the package (dart test, 252 tests) still passes.

FPOCTSMP-8

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

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[DaNiELChIoRo]

Closing in favor of #40, opened from a different identity.