fix: validate unsupported template placeholders

[RoopakBRK]

This PR adds a small validation improvement for query and document templates.

Issue - When an unsupported placeholder is used in a template, for example {domain}, Python currently raises a KeyError when formatting the template. This doesn't make it very clear to the user that the problem is with the template they provided.

What did I change ?
I added handling for the KeyError raised by template.format() and converted it into a ValueError with a more helpful message that points out which placeholder is not supported.

Testing
I ran the targeted tests for extract_texts, and all 9 tests passed. I also ran:
mise run test
mise run lint
mise run typecheck
git diff --check

File Changed -> sie/packages/sie_server/src/sie_server/adapters/_utils.py

Summary by CodeRabbit

• Bug Fixes
  • Improved error handling when a template contains an unsupported placeholder.
  • Error messages now identify the problematic placeholder and retain the original error details for troubleshooting.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 9a7e8d3 and 3bc0870.

📒 Files selected for processing (1)

• packages/sie_server/src/sie_server/adapters/_utils.py

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📝 Walkthrough

Walkthrough

The template text extraction path now reports unsupported placeholders with a descriptive ValueError and preserves the original KeyError.

Changes

Template error handling

Layer / File(s)	Summary
Unsupported placeholder reporting packages/sie_server/src/sie_server/adapters/_utils.py	extract_texts converts missing template placeholders into a descriptive ValueError and chains the original KeyError.

Merge Risk: ⚪ Minimal · up to 3bc08

This change converts unsupported template placeholders into clearer validation errors. No actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: validating unsupported template placeholders.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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