[repo-assist] perf: XmlDoc.escapeXml fast-path + UniqueNameGenerator ToLowerInvariant dedup

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Summary

Two low-risk design-time performance improvements in Utils.fs:

1. XmlDoc.escapeXml — IndexOfAny fast-path

Before: every call to escapeXml called .Replace() three times, allocating two intermediate strings even when the input contained no XML special characters (&, <, >).

After: a single IndexOfAny(['&'; '<'; '>']) check gates all three Replace calls. When the string is plain text (the common case for OpenAPI summaries and descriptions), zero intermediate strings are allocated and the function returns the original string reference unchanged.

// Before — always allocates 2 intermediate strings
let private escapeXml(s: string) =
    s.Replace("&", "&").Replace("<", "<").Replace(">", ">")

// After — fast path when no special chars present
let private xmlSpecialChars = [| '&'; '<'; '>' |]

let private escapeXml(s: string) =
    if s.IndexOfAny(xmlSpecialChars) < 0 then
        s
    else
        s.Replace("&", "&").Replace("<", "<").Replace(">", ">")

This function is called once per summary, once per description, once per parameter, and once per return doc during schema compilation. For a schema with 50 operations × 5 parameters on average, that's ~350 calls — previously always allocating, now virtually free for plain-text content.

2. UniqueNameGenerator.findUniq — precompute lowercase key

Before: newName.ToLowerInvariant() was called on every recursive iteration of findUniq, even though only the numeric suffix changes between iterations.

After: prefix.ToLowerInvariant() is computed once in MakeUnique and passed as prefixLower to the recursive function. On collision, only the (small) integer suffix is appended to the already-lowercased prefix.

// Before — ToLowerInvariant on every iteration
let rec findUniq prefix i =
    let newName = sprintf "%s%s" prefix (if i = 0 then "" else i.ToString())
    let key = newName.ToLowerInvariant()
    ...
    | true -> findUniq prefix (i + 1)

// After — one ToLowerInvariant per MakeUnique call
let rec findUniq prefix prefixLower i =
    let newName = if i = 0 then prefix else $"{prefix}{i}"
    let key = if i = 0 then prefixLower else $"{prefixLower}{i}"
    ...
    | true -> findUniq prefix prefixLower (i + 1)

member _.MakeUnique methodName =
    findUniq methodName (methodName.ToLowerInvariant()) 0

Test Status

✅ All 389 unit tests pass after the change.

SwaggerProvider.Tests  Total: 389, Errors: 0, Failed: 0, Skipped: 1, Time: 1.017s

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Pull request overview

This PR introduces two design-time performance optimizations in Utils.fs aimed at reducing allocations and repeated string operations during schema/type compilation.

Changes:

• Add an IndexOfAny fast-path to XmlDoc.escapeXml to avoid unnecessary string allocations when no XML special characters are present.
• Optimize UniqueNameGenerator.findUniq by precomputing the lowercased prefix once per MakeUnique call to avoid repeated ToLowerInvariant() work during collision resolution.

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