fix: prevent silent failures on invalid placeholder syntax

[sushant-suse]

Fixes #233

What changed:
Added strict syntax validation for configuration placeholders. The application will now explicitly raise an error when it encounters malformed or orphaned curly braces (like {server.name or }} hello {{), instead of silently leaving them unresolved.

How it works:

• Introduced PlaceholderSyntaxError.
• Added _validate_brace_syntax to the PlaceholderResolver to track brace pairs mathematically.
• Updated AppConfig and EnvConfig Pydantic models to catch and format the new exception for the CLI.
• Added unit tests to ensure missing start/end braces and inverted escapes are properly caught.

[github-actions]

Coverage Report

For commit cb0f13c

Click to expand Coverage Report

  Name                                           Stmts   Miss Branch BrPart  Cover
  --------------------------------------------------------------------------------
+ src/docbuild/models/deliverable.py               180      1     22      0  99.5%
+ src/docbuild/cli/cmd_check/process.py             58      0     22      1  98.8%
+ src/docbuild/models/manifest.py                  111      1     12      1  98.4%
+ src/docbuild/utils/pidlock.py                     79      1     14      1  97.8%
+ src/docbuild/cli/cmd_config/list.py               26      0      8      1  97.1%
+ src/docbuild/cli/cmd_validate/process.py         178      5     52      4  96.1%
+ src/docbuild/cli/callback.py                      35      0     10      2  95.6%
+ src/docbuild/utils/concurrency.py                 69      3     18      1  95.4%
+ src/docbuild/cli/cmd_cli.py                      110      3     16      3  95.2%
- src/docbuild/config/xml/stitch.py                 47      5     12      0  88.1%
- src/docbuild/cli/cmd_metadata/metaprocess.py     215     26     66     13  82.6%
- src/docbuild/cli/cmd_config/validate.py           21      2     12      3  78.8%
- src/docbuild/cli/cmd_check/__init__.py            18      5      2      0  65.0%
- src/docbuild/cli/cmd_build/__init__.py            13      5      0      0  61.5%
- src/docbuild/cli/cmd_metadata/__init__.py         27     10      2      0  58.6%
  --------------------------------------------------------------------------------
+ TOTAL                                           3068     67    746     30  97.0%
  
  47 files skipped due to complete coverage.

[tomschr]

Great work! 👍 I have only two questions. Maybe you've added two files by accident. I guess you want to remove them. 🙂

Thank you!