fix: ensure MCP tool name prefix starts with a letter for Gemini compatibility

[octo-patch]

Fixes #874

Problem

_addUniquePrefix generates a 6-character random base-36 string via
Math.random().toString(36).slice(2, 8). Base-36 uses digits 0–9 and
letters a–z, so the string frequently starts with a digit (e.g. 4fzyo8).

The resulting tool name (4fzyo8_read_file) is then sent to Gemini as a
FunctionDeclaration.name. Gemini enforces:

Must start with a letter or an underscore. Must be alphanumeric
(a-z, A-Z, 0-9), underscores, dots or dashes, with a maximum length of 64.

This causes a 400 Bad Request / INVALID_ARGUMENT error for every Gemini
request when MCP tools are registered.

Solution

Prepend the fixed character m to the random segment so the prefix always
starts with a letter:

// Before
return `${Math.random().toString(36).slice(2, 8)}_${base}`;

// After
return `m${Math.random().toString(36).slice(2, 8)}_${base}`;

This is the minimal change needed: uniqueness is unchanged (the random part
still contributes 6 random base-36 chars), removeMCPToolNamePrefix still
works correctly (it splits on _ and drops the first segment), and all other
providers are unaffected.

Testing

• Verified that m${Math.random().toString(36).slice(2, 8)} always produces a
  string starting with m (a letter), satisfying Gemini's constraint.
• removeMCPToolNamePrefix('m4fzyo8_read_file') → 'read_file' ✓

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud