fix(point): escape line protocol control characters

[bednar]

Supersedes #49 and implements the escaping approach proposed in this review comment.

Proposed Changes

• escape actual newline, carriage return, and tab characters as literal \n, \r, and \t sequences before serializing structured line protocol
• apply the shared escaping rules to measurements, tag keys and values, default tags, field keys, and string field values
• cover both Point and DataFrame write paths while leaving raw line-protocol strings unchanged
• preserve the existing serialized representation of literal backslash sequences
• retain point-tag precedence over default tags and continue dropping all-null DataFrame rows
• add regression coverage for exact serialized output and HTTP request bodies

This prevents control characters from splitting structured writes into additional line-protocol records and aligns the Rust client with the Java, C#, JavaScript, and Python client behavior described in #49.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.73418% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.32%. Comparing base (b98caa8) to head (44b2856).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/write_dataframe.rs	98.48%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   82.74%   83.32%   +0.57%     
==========================================
  Files          10       10              
  Lines        1971     2027      +56     
==========================================
+ Hits         1631     1689      +58     
+ Misses        340      338       -2     

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[Copilot]

Pull request overview

Rejects unsafe control characters in structured line-protocol writes to prevent malformed or split batches.

Changes:

• Adds shared validation and a dedicated error variant.
• Applies validation to Point, default-tag, and DataFrame writes.
• Adds regression tests and release notes.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/point.rs	Validates structured Point values.
src/write_dataframe.rs	Validates DataFrame serialization.
src/error.rs	Adds InvalidPointData.
tests/point_tests.rs	Tests Point validation and literal escapes.
tests/write_tests.rs	Tests default-tag behavior.
CHANGELOG.md	Documents the fix.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

[Copilot]

🟢 Approval recommended

The shared implementation covers the stated structured-write paths with appropriate regression tests and no unresolved correctness issues.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Balanced