[PLAT-8748] Truncate strings longer than maxStringValueLength

[nickdowell]

Goal

Truncate app-provided strings that may lead to oversized payloads - metadata strings and breadcrumb messages.

Changeset

Adds maxStringValueLength to BugsnagConfiguration.

Implements string truncation functions in BSGSerialization.m.

Implements -[BugsnagEvent truncateStrings:] to truncate breadcrumbs and metadata, called by BSGEventUploadOperation immediately prior to upload.

Updates BugsnagMetadata and BSGSerialization.m to avoid duplication of sanitization logic.

Testing

Adds unit tests to verify truncation of strings, including checks that composed character sequences are not sliced.

Amends barebones test to very truncation.

[github-actions]

Bugsnag.framework binary size decreased by 88 bytes from 774,680 to 774,592 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.2%    +616  +0.2%    +616    __TEXT,__text
  +0.5%    +132  +0.5%    +132    __TEXT,__objc_methname
  +0.5%     +96  +0.5%     +96    __DATA,__cfstring
  [ = ]       0  +0.7%     +88    [__LINKEDIT]
  +0.1%     +72  +0.1%     +72    __DATA,__objc_const
  +0.4%     +65  +0.4%     +65    __TEXT,__cstring
  +1.7%     +56  +1.7%     +56    __TEXT,__gcc_except_tab
  +0.4%     +24  +0.4%     +24    __DATA,__objc_selrefs
  +0.4%      +4  +0.4%      +4    __DATA,__objc_ivar
  -0.3%      -8  -0.3%      -8    Binding Info
  -0.2%      -8  -0.2%      -8    __TEXT,__unwind_info
  -0.0%     -16  -0.0%     -16    String Table
  [ = ]       0  -0.2%     -16    __DATA,__bss
  -1.0%     -32  -1.0%     -32    __DATA,__const
  -0.0%     -64  -0.0%     -64    Symbol Table
  -1.5%    -164 -11.3%    -148    [__DATA]
  -6.0%    -861  -6.0%    -861    [__TEXT]
  -0.0%     -88  [ = ]       0    TOTAL

Generated by 🚫 Danger