Mark all existing options as non-updateable

[sirzooro]

Initial changes to allow changing selected options at runtime. This is needed for Cryptex implementation.

[Copilot]

Pull request overview

This PR introduces a constructed flag on Context and uses it to make all existing ContextOptions reject mutation after context construction, establishing a baseline for future “selectively updatable options” work (notably for Cryptex).

Changes:

• Add Context.constructed and set it during CreateContext to gate option updates.
• Make all existing options in option.go return ErrContextOptionNotUpdatable after construction.
• Add/adjust tests to validate the new non-updatability behavior and update a test helper to mark contexts as constructed.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
context.go	Adds the constructed flag to Context and sets it during CreateContext.
errors.go	Introduces ErrContextOptionNotUpdatable for post-construction option mutation attempts.
option.go	Adds constructed-state checks to all existing ContextOptions.
option_test.go	New test coverage for option “set during construction only” behavior.
srtp_cipher_utils_test.go	Marks test-created contexts as constructed to align with new option gating.

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[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.23%. Comparing base (df5f1a8) to head (1842d60).

Files with missing lines	Patch %	Lines
option.go	92.85%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
+ Coverage   84.06%   84.23%   +0.17%     
==========================================
  Files          19       19              
  Lines        1512     1541      +29     
==========================================
+ Hits         1271     1298      +27     
- Misses        134      135       +1     
- Partials      107      108       +1     

Flag	Coverage Δ
go	84.23% <93.10%> (+0.17%)	⬆️

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